hash locked multisig, can one cosigner sign without the pre-image?
Can Alice just sign a transaction without revealing H, and just hand over the partially signed transaction off chain to Bob so he can reveal H + add his signature, make the transaction final and spend it?


Certainly. Of course the software will need to know how to handle that data, but it is not that hard to do.

The pre-image is not used in signing in any way; it is just additional data required by the script in order for the script to validate correctly.