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Try hammering a similar sized Torx bit in.
+1 for this technique. Although you have to be careful with smaller bolts as if you hammer in a torx bit, you can crank it a lot more and end up snapping the bolt (just like I did on the seized bottle cage mounts on my beater CX frame). Those look beefy enough that they should be ok. Just get lots of wd40, penetrating oil or whatever in there to help it ease it out!
Bon chance.
Thanks guys for the quick responses! I will get cracking with it this afternoon. I will let you know if it worked.