Some younger people would say your answer is disk brakes
Foe steel frames i know of no current removable boss bases. QBP/Problem Savers use to offer a repair boss kit that sort of could work but not at all what i would suggest. Know any one with a mill that likes after work beer? Not hard to draw out and make. Andy
Got lost in the football game and forgot to add- Back in the early 1980s Moots offered clamp on canti bosses so one could do what you ask for. Change from 700 road to 584 MtB. Andy