I had a mess-around doing something similar (sort of free-moving entities that you could navigate between and walk in) and I did it using the parenting technique. I had a world root that the character was normally parented to, then would swap parents based on the "body" that was active (based on collisions).

I think what you're doing is a slippery slope though (pun not intended!). My number one development rule is: Do not implement something you don't need.

That's time you could be spending on things you do need, time you'll come to wish you hadn't wasted. That's complexities and scope for additional bugs that could affect normal operation, bugs that you'll curse you even allowed to create.

It's a very tough rule to learn.. but following the test-driven development methodology in application development has shown me that it's a great way to work. Doubly so with game-programming which I feel has a bit more of an "artistic" or "experimental" nature to it.