Hmm, good idea. A member here told me once that the reason why some games are in mono is because the developer didn't care to/didn't have the funding to buy a real audio processing kit from SNK. So they instead just created real tracks, and resampled them in PCM BGM.
Yumkobo, Aicom, and of course Noise Factory all have mono BGM. Their SFX are still stereo when appropriate.
For the one's that I have played, the following games have MONO BGM:
Blazing Star
Matrimelee
Metal Slug 4
Metal Slug 5
Prehistoric Isle 2
Pulstar
Sengoku 3
Games I would expect to have mono BGM, but I don't own:
Rise Of The Dragons because it is an Evoga/Noise Factory effort