Yeah, Russian Far East shouldn't really be called Siberia... especially the Southern part. My bad.
I think that maybe we should see support from Outer Mongolia? Possibly the Pan-Mongolian State supported by Semyonov actually manages to come into being, at least temporarily?
A Pan-Mongolian state supported by White Russia and Japan as a weapon against both the Bolsheviks and the Chinese is an interesting possibility. It would also make securing Far-Eastern independance a lot easier... Green Ukraine would probably be a Japanese/American aligned state. I can see it gradually becomming more Pro-American though; there are a few territorial conflicts with Japan that would need to be resolved, and especially when China invades what remains of Manchuria I could see the Green Ukraine becoming a bit worried.
We could also see further Ukrainian settlement of the region following the defeat of Ukraine-seperatist movements. An influx of White Russian refugees is also possible. Green Ukraine would inherit the remainder of the Russian Pacific fleet, which would be key to its defensibility.
I drew up a rather crappy map:
Large/Strategically Important Population centers in the Green Ukraine:
1. Permskoye (OTL Komsomolsk, Capital as it's the most defensible city)
2. Nikolayevsk (Important in defending the mouth of the Amur River)
3. Okha (source of Petroleum)
8. Nakhodka (fishing settlements expanded into an Ice-free port by Ukrainian/White Russian refugees)
9. Vladivostok (Single largest city)
10. Semyonovka (Military production center)
11. Lesozavodsk (Sawmills and a railroad connection)
12-13. Vyazemsky and another unnamed town (Food production and transport hub as connected to the Trans-Siberian Railway and the Amur River)
14. Birobidzhan (Administrative center)
15. Alexandrovsk (Large population center)
16. Gondatti
17. Unnamed town/Uda (Timber and fur production)
Please disregare 4-7. The southern half of Sakhalin is actually part of Japan at the time, I forgot.
Also, I forgot the large coal-producing city of Artynom (which is named something else ATL) and the lead mining town of Tetyuhke, among a few other fishing towns.
The Economy of the Green Ukraine is based off Coal/Oil Production, Timber, metal production (including ferrous and prescious metals... this area actually contains Russia's largest gold reserves) and a military-industrial complex.
The agricultural needs of the Green Ukraine is met by Fishing, growing Grains, Soybeans, Potatoes and Vegetables. Most of the growing happens in the southern regions. The nation is self-sufficient agriculturally.
Major trading partners include China, Japan, Canada and the United States.
So, the nation could have a large population, a prosperous economy, and a potent navy.
Of course, this is presuming the nation survives...