Using Qt as a Cocoa stand-in sounds complicated. Since Darling already uses its own loader and syscall hooking to provide compatibility for the program, why not do the same for a framework like Cocoa?
I think zw meant that the Cocoa binaries can't be distributed with Darling. That's not a major problem for users that have a copy of OS X. Darling can allow users to supply their own.
The potential issue you refer to would also prevent implementing the Cocoa API with Qt.