If you take the pressure off it by pressing down on the arm, you should be able to turn it out by jamming an old screwdriver into the hex.
You can probably match the thread, but not the shape of the head. If you can find it a stainless steel screw would be much more preferable.
BTW- if you ask at the LBS, odds are that eventually they'll replace a crash damaged brake from which the original part can be scavenged.
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