I just tried this out:
basic rule to join the Empire is to have less then 201 total dev (because basically for every dev you need one opinion with the emperor to join)
Now in the beginning poland starts with 183 individual development which would be enough to join but their vassal pushes your dev up to 201 wich is too much.
what I did is exploit dev in all your provinces which will eventually reduce your dev below 200, then I think I needed 180ish opinion with austria which should be doable if you gift them money etc.
all you need is now "bad" RNG to get the PU over lithuania as late as possible so that you can join bevor getting lithuania.
That is quite nifty!