You've read through some older posts, so perhaps you know about the Blitzsafe adapter many of us are using. Blitzsafe makes one specifically for use in BMWs and Minis, and it allows the Pioneer XM tuner to be used in conjunction with an M-bus CD changer (Alpine).
I have heard that Alpine makes a CD changer that can read CD-Rs with MP3s on them, and I would assume that your stock headunit can control it. Failing this, perhaps there's a straight-up MP3 device that uses M-bus, which would make your installation a piece of cake.
Conforti chip, Evo airbox/K&N filter, UUC short shift kit, oil pan baffle, Hella H4 lights, OEM glass sunroof
sold: '99 M3 Coupe
Airbox baffle removed, ZKW ellipsoid headlights, UUC short shift kit, BMW x-brace, debadged
'96 Jeep Grand Cherokee Laredo V8