I know I'm a bit late to this thread but I stumbled on this while doing a Google Image search for YM2203 circuit diagrams :D  I've recently gone back to this project as I've learnt a lot about circuit design over the last year as I've been working on my own custom PC keyboard using a Teensy.  I've made a number of improvements based on feedback I've had and stuff I've learnt and I'm almost ready to get a prototpye PCB made.  Responding to some of the other suggestions in this thread, the reason I went for the YM2203 is that it uses the exact same register arrangement as the YM2149 but has some extra registers for the FM channels.  The pinout is slightly different and the ST only uses 2 of the 3 control lines (the 3rd is just held high on the ST), hence the need for the extra logic to translate the control signals from the ST to something the YM2203 should understand (basically converting 2-bit control to 3-bit).

p.s. how do you get it to display attached images in the post?