You could flash with MSX-DOS2, and use as DOS2 cartridge with the SCC chip available. Ideally using a version that can be disabled with a keypress @ boot time. Then at some other time, re-flash with a game ROM and use that - possibly SCC sound included.
On the NMS8245, internal slot 3-1 isn't used. Circuitry-wise an internal slot isn't difficult, but actual construction might be. Especially if you have as little space as inside a '8245. In this machine you could route a flat cable from the back slot under the mainboard, to a (Konami-sized?) cartridge under the keyboard.
Of course some wires on that flat cable would have to be re-wired or go into some add-on circuitry. And there isn't any rule that says you can't have slot 3-1 on the back and slot 2 on the inside...
Personally I'd pick another machine for such a mod. More internal space = more options what cartridge(s) to plug in.
