Reassigning the incoming signals as you suggest would take about as much time as assigning the channels on the 14MZ to conform with a student's transmitter.
No it wouldn't. After you assign the channels, you would then have to move plugs on your receiver. I am almost 100% sure that adding the ability to remap the student channels would be a trivial programming task for the Futaba folks working on the software and it would be far more useful. If someone came to the field and wanted to fly my aircraft, I could jump into the trainer function, assign trainer channel numbers to my channel numbers and be off. Then if tomorrow, someone else, with a different transmitter wanted to try it, within 30 seconds I could have them flying it as well.
Please, please, please, add it as a feature request. I am sure it would be simple.
Regards, Guy