An update!
After months of nothing, hooked up an Arduino to a C64 connector, picked an organ sound and wrote a very rubbish program to play the first lines of a well known tune.  Have a look and listen at:

Arduino plays an SFX Sound expander

I know its a bit basic, but its a start.
Feel free to comment, but please make them constructive!

I know this is an old thread , but I've been working on a Dual sid + FM ( 3526 and 3812 )  music driver for a while now. Should have something out there very soon. Also in the process of modifying the C64 version of Bubble Bobble to play the actual arcade music if you have both a Commodore 128 and the FM cart. I did find complete documentation for the FM chips and the writing/reading technique as used by you guys is correct.
Will keep you progress posted,

Did you have to strip out any of the FM calls in the Bubble Bobble music?  I noticed it hits a lot of the registers kinda randomly.  (not as badly as Ikari Warriors though)