i added a remote reservoir when i had a line leak and didn't know it until i lost my brakes on the 11 power tour
it's nice to be able to check fluid level when i open the hood
i've had an underfloor mc on 58-59 trucks since the 70s never had a problem until i upgraded my brakes, and had carpet glued down over the mc cover.
back to his question, i don't see a problem with drilling the mc if it's done right.
you'd need a leak proof mc cover and the right lines.
don't like the reservoir hanging on my shaved firewall, but here it is