Yes it would be quite easy to do but seeing the display may be the issue. You'd be almost directly above it. I know with seat all the way back I can't see that pocket but directly under the shelf might be an option.
The good thing about that is you can leave the original radio and even keep it powered up for clock and the plastic below would be easy to find and replace if you ever wanted to sell it.
A wiring harness could be extended like you said and be connected. If you're moving into the rear you may rip off buttons trying to slide by it but you could just be careful.
I may do this myself. Good idea. I'm gonna add a subwoofer when I get off my lazy behind. That would make it easier with an aftermarket unit.
The angle of the plastic might be an issue but if you mount it good it will be fine.