Polonize Belarus? Depends on how hard they try I think.
If they try to Polonize it, IMO they'll provoke a backlash. If they don't actively try to Polonize it, then economic links going west into Poland proper, the growth of literacy and the introduction of television down the line (if Poland still has Belarus by the time it's relevant) could well lead to substantial Polonization.
In OTL, the Poles did fairly well at getting on with and integrating the Belarusians in Poland (insofar as they could given the level of development of the border lands - given another generation of development from a 1939 where WW2 doesn't happen, things could have gone either way), but very poorly with the Ukrainians. A big part of that, as I see it, it because they tried harder with the Ukrainians. Though there are many other factors from bad prior history, the decisions of some rather unpleasant actors on the Ukrainian nationalist side and from external support from Czechoslovakia and the Soviet Union that fuelled the poor situation between Poland and its Ukrainian minority.
One can certainly imagine a situation where the Poles work hard to make the Belarusians into Poles, alienate them, enabling unpleasant nationalists to gain power, who draw on support from Lithuania and the Soviet Union, all in the same timeline as the Poles didn't push Polonization of their Ukrainian minority, denying opportunities to their unpleasant nationalists, meaning Soviet and Czechoslovak efforts to destabilize Poland fall on infertile ground in Polish Ukraine... It's hard to know what would really happen, but one can argue that a straw/camel's back dynamic existed.
This is all just my opinion. Much depends on how you interpret
the why of how interbellum Poland had better relations with its Belarusian minority and worse relations with its Ukrainian minority.
fasquardon