After reading over the Wikipedia page on that chip it really does seem like the perfect candidate. Tiny, just enough features to be useful, has a DAC already in it. \o/
Actually, thinking about it further, other (surface mount) chips could be used as well, if you put them on a PCB and put that PCB on top of the CPU. There should probably be enough space there to accommodate the chips. However, this would require a small custom PCB and SMD soldering skills. Anyone who can hold a soldering iron should be able to install a YM2413 with a couple of wires, and put it in the right side pocket of the DMG case.
The bad aspect of the chip is that only lets you define one completely custom instrument at a time. (You also have 15 presets to choose from.)
Did any games make use of it?
It was used in Sega Master System, (in some games) and the NES VRC7 chip is a derivative of it. For inspiration:
could this chip also act as a sampler?
No, not really.