Honestly, I don't think it'd take all the much time to explain in a new film. I mean, just throw some cyborg legs on him, and a lot of people would put their own pieces together of the puzzle. That's how they did things in the original trilogy. They didn't explain how Kenobi was able to come back and instruct Luke as a ghost, people sort of figured it out.

This is why I'd much rather see him in that rumored Maul/Kenobi spin-off movie than any of the numbered ones.

Yeah, how freaking sweet would that be? Then they could use the numbered films to move forward without having to bring in any Prequel characters (which would be weird and raise the question of what they Hell were they doing during the OT?), but still give the character an awesome big screen return.