Thanks everyone for the input, it has been extremely helpful.
So yeah, I've determined that venturing into the hold just wont be that viable, excluding maybe for crew rest or additional storage or something like that.
I'd love to be able to tell you exactly what my idea is - I really would, but I'm sure you can all understand why I wont. However, I'm going to go ahead with the planning of it, (excluding the cargo bay part) and hopefully within a short time I'll be back here asking for feedback on the full idea.
Oh and it's okay, I'm not going to be messing about with the cycles of a 747, it a long range aircraft - I'll leave it at that
Thanks again, for all your input.