Interesting question, not so stupid but sort of pointless maybe. I can't answer for you on your Montana [or any other outdoor device at present, 'cos i haven't got one to try with]. But it certainly works on a nuvi to run a txt command on the microSD to delete maps from the Map folder. However, a command to copy from the Spares folder to the Map folder seems to always fail.
The reason I say its sorta pointless is that you need access to a computer to write the update.txt file and add the ldr.bin in any case.