Yes, just like on the PC, you can use a hub to expand the number of ports. Presumably to a maximum number of 7.
What I couldn't tell you for sure is if each controller consistantly keeps it's controller number between reboots.
PS3 supports at least 8 simultaneous local controllers... not that any PS3
games do that...
And no, they do not keep their numbers between reboots. In fact, I haven't been able to figure out how it determines the number assignment. Sometimes it seems to be whichever gets powered up first... Sometimes it feels completely random.
That said, all you need to do is hold the PS button to bring up the menu to change controller number assignment. Once a session, max.
Are you putting a PS3 inside a cabinet and wiring an already existing control panel to it? You should know wired controllers cannot power-on the PS3; only Sixaxis and DS3 controllers can do that. You'll want to keep one handy if the PS3 is in a hard to reach place.
More importantly, if this is the case, why put a PS3 in a cab? Among the
many reasons not to is the fact that the majority of PS3 games use the analog sticks, not digital controls.