Yes, something like this. As I said, I'm not sure.
Another angle I'm missing is the choosing of side for the rest of the HRE members. Some of them hate the Emperor? - take the opportunity to leave with you with a relations bonus to Germany.
I think that'd be great - as long as the Emperor gets a CB that can force them into the HRE again (and they shouldn't be able to leave again for 50 years). Otherwise, the current frequency of states leaving the HRE is pretty good. I'd also welcome if more nations outside the HRE tried to join, like a weakend Provence, the Teutonic Order or East Frisia (they used to join almost every game before, but in the recent patches they rarely join). I'd also go a step further - provinces that aren't part of the HRE do not get the HRE bonuses (-development cost, -2 unrest, etc.). That'd be incentive enough for states to add provinces to the HRE.