How about putting the MacBook Pros "heart and meat" (logic board and the rest of the innards) inside the Ti PowerBook G4's case (without iSight, I do not care about iSight)?
Nope. The MBP is
bigger in every way possible. The screen is larger (I believe the screen on the tibook was 15.2 or something, the MBP 15.4), the MBP is thicker, the "flap" holding the screen is bigger, it has a different clasp (I seem to remember the ti having a single one, the mbp has two clasps), the ports on the tibook were placed in the back behind a hinge (not on the side as on the MBP) and so on. It wouldn't be possible to fit the motherboard of the MBP into the tibook, as you would have to redesign it to fit (for one, all the ports would have to go on the back, and afaik, the tibook only had two speakers, not three as on the MBP, which would mean, you would have to redesign everything, including the airflow through it in order to make it fit. This includes the keyboard and so on.
However, look on the bright sides: The Tibook sucked huge arse. I loved it, but quality wise it sucked. It was very, very flexible, and with the two minute hinges in the back, it didn't take much to wreck them. People (me included) had problems with both pitting and flaking caused by the poor paint job. The white rim flaked on mine (it was carbon fiber, but still flexed, causing the top layer to flake) to reveal the abysmal grey primer underneath. The palm rests (as on my MBP) pitted to reveal another shade of primer underneath that. It truly sucked, and the Tibook in appearance wasn't all that "Ti" as the whole thing was painted in "titanium color", instead of letting the real ti shine. I still wonder what on earth was in that paint.
To this date, I think of the TiBook as the turning point of apple: The day the tibook went public, was the day the Apple began it's spiral with dwindling quality.
Sorry, that was way more than you asked for.
what about using magnesium and alloy like the panasonic toughbooks?
The Tibook (at least, propably also the MBP?) used magnesium as the support stringers for the screen.