You could try to get the same dynasty as Bohemia or Austria (elective monarchy op) and claim their throne to enforce a PU. Then once you inherit them, you can westernize for free. That would take forever, though.
I've had royal marriages with both, so I will keep that in mind, thanks for suggesting it. I mean it looks like westernization might not really be an important goal in the near future anyway, and I might end up attacking Bohemia at some point anyway too, but maybe circumstances will make what you're suggesting a prudent alternative.
For most of the game (just under 100 years so far) I have built up a block of close allies: Pomerania (which up until 5 years ago has been fairly strong), Bohemia (which sticks out into me like a cancerous blob aka Silesia, which I want), Austria, and Sweden. I have royal marriages with all. Somehow they have all allowed me to court them to "buddy buddy" status all at the same time and so most of my borders are very secure and neither Muscovy or the Ottomans have bothered me, due to my relatively large and immediate possee. I have been able to annex my vassals, castrate the Teutonic Knights and Hungary, and form the Commonwealth in relative peace, I think there was basically only one "make it or break it" war where I had to break the treaties between the Teutons and the Hungarians. After that everything was a methodical march towards the Commonwealth. I think a war between my group and Muscovy is brewing, though, I just noticed that they are allied with the Livonian Order, what's left of the Teutonic Order, and a bunch of other states. That's going to be fun, but I'm not sure if I should take Crimea first.. It looks like it's free for the picking and so I might have to step in before somebody else does, before I march on Moscow.
So we'll see, but I think I have enough firepower to castrate Muscovy soon, and after that I will only have to worry about the Ottomans, unless any of my allies turn on me, which seems unlikely as they seem to love me. Maybe I'm getting too cocky for my own good, but I am a rather large blob. My diplomats are already making new friends in the empire, with France, England, for some reason Norway, Naples, and basically anyone they have time for.
Don't worry too much about Westernisation as Poland. Eastern tech isn't bad at all. When I did my achievement run as them, including "Poland Cannot Into Space" (i.e. max out tech), I never bothered, as maintaining tech parity wasn't an issue. If you happen to control the provinces for the decision, take it, but it's no big loss if you don't, and you should never be far enough behind to do it the normal way.
I have been focusing on technology and my economy so that I can form the Commonwealth sooner rather than later without too many problems.. and I guess it worked very well, because when I last checked the technology map, the only countries that were green were me and my ally Bohemia. Which made me think exactly that "Maybe screw westernization?"
What might very well happen is me turning on Bohemia after I'm done conquering Moscow and Constantinople, I think it will be easier to attack parts of the empire without getting your butt kicked then.. I definitely want Silesia, right now it's a part of Bohemia, so while Bohemia's a good friend, they definitely have something I want.. So I don't think we're staying friends forever.
edit: awww crap Muscovy is also allied with the Ottomans. I might have to sit this one out and wait for more favourable circumstances...