It is an ardupilot limitation as far as i know, MP would have no probs in supporting override above channel 8. From what i understand from 3.7 ardupilot will support override up to 16 channels, so i guess we will have to wait a couple more months untill 3.7 is out. I asked in the 3.6 forum some clarification but didn’t get an answer yet.
If you search rc override in ardupilot github repository you’ll find what i just wrote about 3.7 support.