Here is a Celica GTS picture:
Here is a walkthrough for a tranny fluid change on a Celica GTS = fundamentally the same:
http://www.newcelica.org/forums/showthread.php?t=121104
Notice that one of the pair of bolts holds a clutch hydraulic hose bracket.
These MAY possibly be accessory type bolts...meaning that you might be able to leave your busted bolt in place without harm!!! In other words they may turn out to do nothing internal to the gearbox and simply be used to hold external items. I'll see if I can find out more...
In the pic above you can see a large allen head bolt to the left of the indicated fill plug...just past the pair of bolts noted above....this can also be used for fill purposes....and apparently is on some Toyotas where obstructions might make the other location too hard to reach.
Hope things work out for you here.