that qml keyboard is pretty cool, so most likely one could make versions of most qml apps to use it. But I doubt that one can replace default vkb with it. Well in theory everything is possible as it is just software, so with enough people and effort transparent vkb is possible, but I doubt that there is enough incentive for anyone to take on such a big task.

I have a low priority todo list to get transparent vkb in presence vnc, but currently I just don't have time to do anything.