The block was already bored over once, so I think it's already at 100mm. But it had very low miles when I spun the rod (literally about 100 miles... it spun on the dyno during break-in because I re-used an oil cooler that apparently still had debris in it from the previous motor's spun bearing).
After that motor died, I basically sat on it for a few months before deciding just to buy a new shortblock from Subaru instead of going back to the machine shop. There was a question as to whether or not the crank would be salvageable, so I figured if I was going to have to buy a new crank and pay for assembly, I might as well just get a new shortblock (STi blocks were only like $1300 back then).
Anyway, since I didn't use the motor, it got packed up in a giant tupperware container with some oily rags, so it shouldn't be rusted at all. But I haven't actually looked at it in years, so no guarantees.
The only concern is that the coating on the piston skirts appeared worn, so there may be the possibility of some cylinder wall gouging. I would guess it would be fine with a light honing and new piston rings, but you'd need a machine shop to check it out and tell you if it needs boring. If it needs boring, like you said, it's probably useless because going to 100.5 is too much... you'd have to sleeve the block.
Anyway, it sounds like you've got a particular plan for boring over 99.5, so my block probably won't work you guys, but I figured I'd mention it anyway. Let me know if you want to check it out anyway.