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...there was a period of time when bikes came with a recessed front brake and a regular nutted rear. I have several sets of brakes from this period, and I've had some frames set up that way as well. I assume the frames came that way, and were not drilled in the fork by a previous owner, but the brake sets kind of bear this out.
I have drilled a rear brake bridge, very carefully, to enlarge it for recessed brake once, on a frame that already had an opening in the fork for recessed. It was kind of anxiety inducing, so I have resolved not to do it again. I have also done that thing where you drill out the threads in a recessed nut, to make a sleeve for the front brake, and use a regular nutted one. I've done that several times, and it caused me no anxiety at all.
The problem with drilling the rear brake bridge is that, unless you have a right angle drill, there isn't enough room for the drill between the brake bridge and the seat tube. So you have to go in from the rear, and this means you enlarge that side of the bridge as well. Which is kind of a bodge. Because on that side, all you have filling the hole is the braaake bolt itself, not the recessed nut on the exterior. So it will move, unless you make a modification, like using one of those curved alloy brake spacers on that side.
I wouldn't do it, especially on a frame that nice.