Hi
The masking tool for ALIGNMENT will be ready in the next release (testing it if it works as intended)
For masking creation itself it is best to use an external application that has proper and efficient tools for this (Pshop, Nuke, etc.)
If you just need to solve a moving car, or so, it is not a big issue, as we “compensate” it to some degree in alignment. If you have sufficient overlap, then it is not an issue at all…