I'm glad it was useful!
Unfortunately, I think that the entry in vgmstream.h is just a placeholder. As far as I know, they haven't actually worked out the format yet.
EDIT: I take that back. Two years is a long time.
https://github.com/kode54/vgmstream/blob/44075c4e911d713ad7277a6b2ee4b7cd9572f7ee/src/coding/nds_procyon_decoder.c
https://github.com/kode54/vgmstream/blob/44075c4e911d713ad7277a6b2ee4b7cd9572f7ee/src/meta/nds_sad.c