Given that at this point they spent their time hoping for one, yes. Brandenburg and Saxony at this time generally remained loyal under Catholic Hapsburgs--they're going to defend a Protestant Habsburg to the death. The Elector-Palatinate's more questionable, but he's likely to stay onboard. And at this time, the number of Catholic secular Princes is very, very low. (And ironically, most of them are actually LESS loyal to the Emperor than the Protestants.)
The Prince-Bishops on the other hand, might prove more problematic, but even then, you might be surprised at the number who'd sign up. While there's going to be problems, the biggest ones are going to be from outside the Empire, not in it.