It seems from my reading, it was easier for a Pole to be accepted as Prussian, than to be accepted as German. If I were to try to write this ATL, I think I would put a POD in the Napoleonic area. Prussia keeps all the lands from the partition of Poland including Warsaw. We see the Prussia national identity include more Polish elements than OTL, by a wide margin. Polish nobility integrates/merges into the Prussian nobility. Polish nobility is important to the Prussian Army.
IMO you are on something here.
German and Polish were still quite nebulous terms at that time.
At that time people easily carried two or three different identities - a late XVIIIth/early XIXth centry person could be a German speaker, Lutheran, yet happily "Polish" as he lived in the PLC.
Hence a prolonged existence of Prussia and somewhat different policies towards non-German speakers and/or non-Protestants could had allowed for the adoption of a Prussian identity by Roman Catholic Polish speakers.
I read somewhere that the post-Partitions forced integration of Polish speaking Presbyterians (Calvinists were Polish speaking szlachta, Lutherans were German speaking burghers) into the Prussian State Church - including use of German in Church - lead some groups to convert to Catholicism out of spite (I've read of a similar instance in Hungary - there the local Calvinists hounded by the Catholic Habsburg authorities converted to Orthodoxy). How true is that - I dunno ...
Look at the Masuren - for a very long time they identified were Polish speaking Prussians.
Identification as Polish by bi-lingual/monoglot Silesians was far from universal.
For the Posen/West Prussian peasants to assume an identity of Polish speaking (and mostly Catholic) Prussians - this coming on top of their regional identity as Wielkopolanie (Great Polish) - is possible.
A different interpretation of (Klein)Deutschland could also help - just as one could be Badenian, Bavarian or Rheinlander AND German, it might had been possible to be a Polish speaker and Poznaniak/Pomorzak/Ślązak (Great Poland, West Prussian and Silesian respectively) and German at the same time. Just as it was possible to be Scots or Welsh and British at the same time.
Destroying the szlachta - either after an AU Russo-Prussian victory in 1807 - or imediatelly after Vienna - would help.
And no Kulturkampf or Hakata ... these simply backfired and only served to fuel rejection of "Germanness" - just like Polish efforts in the Borderlands between 1921 and 1939 did ...
I wonder if the Prussian/German state picked the worst possible route - the repression was
not enough to force assimilation but
strong enough as to foment resentment.
Had a different route been taken - even after OTL Vienna - I can see Posen and Westpruessen as being much more Germanised come 1914. Closer to Upper Silesia, with language/bi-lingualism not being as clear a divider as in OTL. Speaking German was simply what one did when rising in life - Polish/Silesian was for peasants/proletariat. First you have bi-lingualism and diglossia and a generation or two later you end up with "local language revival" movements.