Fuel injection and O2 heaters have some surprisingly large fuses on them, definitely will draw more amps than the head lights.
I would be tempted to just hook everything up see what happens to the voltage at the dash when everything is on (cold engine, heater, lights, all seats, stereo at full volume etc.) If you can't maintain close to 14 volts at the fuse box then just upgrade to a larger alternator then. The higher capacity alternators are the same physical size so shouldn't be tough to leave the fuse out of the seat heater circuit until you get a bigger one. I think you can even get upgrade kits that allow rebuilding the current alternator to more amps.