Sure, you could create other folders and put the extra content in there but to be on the safe side I would put them in the AUDIO_TS folder. You never know if you have a fussy set-top player.
For example an old Alba player I had refused to play The Matrix because of the autorun info and movie player on the disc for pc use. I removed these and all was well.
As the OP did not specify autorun menus etc, storing in the AUDIO_TS folder is easiest IMHO
Is this actually a hybrid disc? I know that CDs with music (CDDA) and data on them are hybrids (Mixed Mode) as the way in which music and data are recorded on a CD are different. However a DVD Video disc is just a plain old data DVD in UDF format. The set-top identifies it by looking for a folder (VIDEO_TS) containing compatible files.