That raises an interesting question. Can a priest or even a lay Catholic adhere to a pope whose identity he does not even know? I don't see how that's possible.
I say yes.
It would be akin to the Western Schism and someone not knowing who the true Roman Pontiff was.
The idea is, "you submit to the true Roman Pontiff whomever he may be" - no heresy involved.
Now submitting to one who you are certain is a heretic as the "true Roman Pontiff" doesn't work either (as I know you are well aware).