You mean, Tver principality, yes?. Because other princelets could not hold even their backyards (and actually welcomed Poles to defend them from... well everyone around - Tatars, Swedes, Livonians... not to mention Tver itself). Well, it is possible, if they successfully claim legacy of Vladimir predecessors, but you need some earlier PoD, maybe around XIII-century Mongol invasion. While they never held the territory - too cold for them, I'd guess - they repeatedly raided it for generations. There was of course, Rus before, if I recall correctly, XII century, but it's unity was doubtful.
If Poland occupied elsewhere (or alt-*Bathory less interested in region, and more concerned about Turks, who subsequently gave sooo much trouble to his successors), there may exist some kind of independent country still. Though, I'd guess it would be subjugated by Sweden in XVII-XVIII centuries. Not sure if they annex them though, after all when there was briefly Poland-Sweded Empire in OTL XVIII, and at dissolution Swedes had no desire to claim Zakraina, even if it was closer to Scandinavia then Poland proper.
More like some kind of puppet state established. Maybe even granted full independence in XX century like Norway and Suomi.