"You miss 100% of the shots you don't take" - Wayne Gretzky
I don't really see any reason why you couldn't. It might make staying in Tokyo a bit harder, but rarely is anyone able to stay in for more than one go around the table with 6 players. We were going to try it at my last game night, but decided to play a 7 player game of The Adventurers: The Pyramid of Horus instead. (Recommended, by the way!)
I think it will be so hard to stay one round in Tokyo that nobody will even try. No more fight, the game will be all about victory points and a few special abilities.
To counterbalance this behavior, you could try to give a defensive bonus in Tokyo (like immunity against the first pawn from each opponent) or increase the reward for beginning your turn in Tokyo (like 5 victory points or a free Power Up card).