The only thing i can think of is a bit dodgy but if you maybe got a slightly bigger torx bit and lightly tapped it in with a hammer just so the torx bit grips you maybe lucky. If not i suppose the only other thing to do would be to cut away the plastic shroud on the back of the steering wheel so you can fit a stud extractor or some mole grips to the bolt head, alough you don't want to go down that route yet.
And just a quick reminder, if your working on any srs system i'ts very important to disconect the battery first. Just make sure you have your radio code to hand and if you have satnav it will need to be shut down properly.
