OK, it looks like I have indeed misinterpreted the OP’s idea.
It seems that the idea is as follows: The Seven Boyars send Sigismund an offer to invite his son Wladislaw as tsar. Sigismund answers that he is flattered but Wladislaw is unavailable. Would Seven Boyars be willing to take Sigismund’s good guy Ostrogski instead, who is extremely wealthy, noble ( whether he is Rurikid or Gedeminid ), Russian-speaking and Orthodox ( the last one is a necessary requirement; so you have to either butterfly away Janusz’s conversion to Catholicism – converting back would probably enrage both Orthodox and Catholics - or butterfly away the death of his Orthodox brother Alexander, who IOTL was probably poisoned in 1603 when he was in his early thirties)?
Unlike
@Augenis I actually think that Alexander Ostorgski ( or Ivan – an Orthodox version of OTL-Janusz) would jump to such an opportunity and it that can be made acceptable to at least a substantial part of Russian nobles and people but as already argued by
@Russian I think it would be an extremely foolish proposition to be made by Sigismund.
First let me deal with issues put forward by
@Augenis:
- The Ostrogski, or Ostrogiškiai, or whatever language you use, were loyalists of the Grand Duchy of Lithuania and supported the Union of Brest, so even though they were Orthodox, they would likely not be accepted by the Russians.
Both Konstantin and his Orthodox son Alexander were opponents to the union of Brest ( in fact Konstantin was the leader of opposition to it going as far as demanding Sigismund to revoke the Uniate bishops chairs and give it back to “true Orthodox” bishops). So I don’t think Russians would have any issues with that.
As far as loyalism goes IOTL they were indeed staunch loyalists of Polish-Lithuanian Commonwealth as they had a very solid reason to remain loyal. IOTL they were extremely wealthy magnates who had an extensive domain centered in Volhynia and directly or indirectly controlled most of Volhynian voivodship and large parts of Kiev voivodship as well as had lands and a very serious influence in other neighboring Podole, Braclaw and Ruthenian voivodships ( say Konstantin Ostrogski himself was marshal of Volhynia and voivode of Kiev for most of his life; both his sons Janosz and Alexander were voivodes of Volhynia as well as starostas of various powiats in Kiev voivodship).
In uncentralized PLC they were practically unopposed leaders of Southern Ruthenian nobility and if not ruled directly had a primary influence in today’s Ukraine west of Dniepr. Of course they were not the hugest fans of becoming subjects of much more centralized Russia that held its nobles in iron fist ( at least if you compare it with PLC “golden liberties”). But becoming a ruler of such a country is a whole other matter – suddenly you begin to really like Muscovite laws that give a Tsar so much power over his nobles ( as it is you who hold the others in iron fist, not the other way around).
So I highly doubt that if Ostrogski have a real possibility to become Russian Tsars that their OTL loyalism would stop them. I also don’t think that would be a large issue for Russians since loyalty to your current ruler is hardly a very objectionable trait (and switching sides in Russian-Lithuanian conflicts is somewhat of a common practice that was never really frowned upon).
2. Both Janusz and his father Konstanty opposed the intervention in the Time of Troubles and meddling with False Dmitry, so they have OTL basis for not wishing to accept the crown of the Russian tsardom.
As far as I know Konstantin at least was hosting False Dmitry I for some time in 1602 which doesn’t look an action of his active opponent.
But even if he indeed was opposed to the PLC intervention I hardly think that means he(or more accurately one of his sons) would have refused the throne if offered. Even if you genuinely think that an Intervention would not bring PLC much good it doesn’t mean that it can’t bring any good to you personally if an intervention is to support your own claim to the throne.
3. The Ostrogski died out with Janusz, so his dynasty would be short-lived anyway.
In order to be an acceptable candidate for Russians Ostrogski has to be Orthodox. Thus either we have a surviving Alexander or Orthodox Janusz.
IOTL Alexander had 5 sons at least one of whom can live if their father survives and thus their circumstances change. Even if they all die like they did IOTL, Alexander died before he was 35 and he probably would have had several more children had he survived.
With Janusz preventing his conversion to Catholicism means dramatically changing his fate. Even if do not wish to consider possible butterflies, IOTL Janusz married his third wife in 1612 with whom he had a son who died in infancy. If he becomes a primary candidate for Russian throne he probably would have to pick a wife from Russian nobility who may well give him a surviving son (or several).
However like
@Russian I really don’t see any arguments for Sigismund to propose such a scheme. It creates a very uncomfortable situation for him or his descendents – a situation in which a ruler of Russia is a major landowner in largely Orthodox an East-Slavic-speaking part of the country as well as a scion of a clan, traditionally managing a large portion of it (and thus having inherited a web of vassals, connections and traditional alliances there).
Even if one of conditions of Sigismund’s support is cession of all lands in PLC to a relative – say if Alexander Ostrogski is Polish candidate for tsardom, he has to cede everything to Janusz (though forcing such a cession would be very tricky to implement without angering other magnates with violation of noble rights) – the situation doesn’t become much better ( a close relative of Russian Tsar virtually ruling over a large portion of country is almost every bit as scary if it was a Tsar himself).
Even if Sigismund has some sort of ironclad guaranties of personal loyalty of Ostrogski candidate (though such guarantees are pretty hard to imagine) situation is bound to explode in the next generation leading to very real possibility of losing an entire eastern part of PLC.
One might argue that situation is not unlike allowing Wladislaw to convert into Orthodoxy. While indeed there are risks of Russian Tsar Wladislaw claiming eastern part of PLC, at least from Sigismund’s point of view it can be outweighed by making his dynasty rule in yet another country. Promoting Ostrogski on the other hand does not seem to give Sigismund anything of value.