It's not generally seen on factory calipers, but would it be OK to powder coat over the surfaces of the mounting points or the sealing surface of the line attachment point?

I'd be leery of PC covering where a sealing washer seals the banjo but what about mounting points?

Mounting points I wouldn't worry about, but as you said I wouldnt want it on the sealing surface where the banjo bolts go, nor in the holes for the actual brake cylinders. If it get in any bolt thread holes just run a tap through and it'll clean it out. My dad just had some done for his CTS-V, I'll ask how they did his.

ETA: also, not a huge deal, it can be removed, but id try to avoid getting into the bleeder screw hole as well.

No, do not PC where it makes contact.  These bolts are at a certain torque.  Over time, the PC will crush, or compact, and change the torque value. It's kind of like putting a plastic washer behind the bolt head.

My old SN95 Mustang had PC'd Cobra calipers.  All hard points and sealing surfaces were uncoated.