Credit for this mod goes to hardMVS. I have done this successfully on my MV1F board, the original author does this on a 4-slot board. It does not matter, as I believe this is possible on all MVS systems and maybe an AES even.
This involves connecting a SPST switch to the #8 dip switch on the board. You can test this without soldering anything by simply flipping the dipswitch while the game is running. On Metal Slug 2 the background effect of the “wavy hot sand” and the music will continue, but the timer, sprites, and input freeze.
If the switch is on when you power up the MVS, the board will not work. Reset the board with the switch off to resume playing.