Jbors, Id suggest the below for the P-BUS.
PC to Pioneer
Folder/CD Name
Track/File Title
This is similar to how the P-BUS currently deals with CD-Text in a CD stacker.
I would suggest the headunit link-to-pc behave in the same way as the stacker, this makes sense instead of a custom controll.
Pioneer to PC
Previous in folder/cd (<)
Next in folder/cd (>)
Rewind (<<)
Fastforward (>>)
folder/cd up (/\)
folder/cd down (\/)
However the head unit I have (Pioneer DEH-P8600) deals with mp3 cds in the HU itself in the following way. See page 29 of manual below.
DEH-P8600 manual 1.85mb
This would be the ideal situation as it would cater for OLD headunit and see your hardware selling into the future as the new Pioneer headunits work in the same way to the DEH-P8600.
*Manual File Size Added