It's not just monkey patching that causes problems, though it certainly doesn't help.

The advantage of going with some sort of transpiler is that you can have two kinds of code within the same process whilst you make a gradual transition. I feel like, it could be made to work, but the boundary would have to be quite heavy-weight. Think less gradual typing and more FFI.

Edit: actually, in depends on which side is doing the calling. Going from checked to unchecked code is a minefield, but if it's unchecked code calling into checked code, it could work quite well as long as the arguments are immutable and don't have methods that can do IO.