The problem with this is that your average PC keyboard doesn't have ";" key. It has "," key, and "Shift"+"," producing ";", which is handled by the BIOS/keyb.

The DOSBox mapper acts at a scancode level. Thus it cannot map anything to ";" because there's no ";" scancode. At best, the mapper could allow you to create macros like "Press Shift -> Press , -> release , -> Release Shift" (which is something it doesn't allow currently).

Keyb/Nokia.sys/Rover.sys acts at BIOS level. Thus it gets to decide which string "Shift" + "," scancode produces.

The only problem with the keyb approach is that some games potentially may bypass it. But those games wouldn't work with international PC Keyboard layouts either.