I found that the trackno, the RIOT uses to sort the tracks of an album is stored in the byte 0x00f9, or possibly the bytes 0x00f8 - 0x00f9 (I don't have an album with more than 255 entries on my machine), of the hd_file_t structure.
Can this structure be influenced during the upload process?
Otherwise, one of the bytes in the rio_file_t structure has the coresponding value, to me 0x02d8 is a possible candidate.
Oops, I think I had something wrong here.
Isn't the RIOT specific block in the rio_file_t structure identical to the hd_file_t structure, but with an offset of 0x0200?
In the case, the trackno has to be placed directly after the size2 field. I will try that.