I've never repaired or maintained a system like that but here's my thoughts. A. I would alow system to cool down before removing the screw,you risk injury from steam. B. That screw head look's exactly like self drilling used in metal wall studs. Odds are the screw has become rusty which mean's it might snap instead of backing out. At best it might screw out but,it's unlikly to reseal if put back in.. Before loosening screw,I'd check threads on another valve then have a mating tap pluse the next larger tap and drill bit as well as both valve sizes on hand.

You make great points about waiting for the system to cool before removing the screw and the possibility of the screw snapping in the radiator.

While I feel comfortable bleeding a normal radiator, I may just bite the bullet and have a professional come out to assess the situation.

I agree with you. If you don't already own taps and bought them,cast iron is tricky to tap threads into. If I'm right about that being a metal stud screw,it's going to be rusted in place. I suspect there's a home brewed gasket between that washer and radiator.

"I may just bite the bullet and have a professional come out to assess the situation."

An excellent idea