The discussion about kitsphrenik clearing stuff out fired a neuron or two.
Old sound cards have some sort of FM chip on them, usually OPL2, occasionally OPL3. I bet you could bodge up an adaptor to drive those from something like an Arduino or other microcontroller, for fully-programmable 2-op FM synthesis madness.
I'm pretty shure the people who made the sammichSID project also got one for the opl2 or 3. You just have to hotair that sucker off the board, as many of them a SMD, but there's many varieties of the YM chips and a bunch are thru hole ones aswell. These soundcards are pretty sought after by the chiptune artist. I once buildt a dosbox with a opl2 card for a dude from Sweden. He paid a good amount aswell.