moving over here from the other thread!
I've got this all working now, many thanks for all your help. The one slight difference from the wiki, in order to get it working easily in PD, was to mount the USB stick into the project folder for my PD project (rather than to a root folder). So that means the path that it's mounting into is /root/Bela/projects/project_name/mnt/usb
For some reason if I tried to mount the files somewhere not in the PD project folder, PD wouldn't find them. I imagine this is a PD issue, or maybe I was just getting the path wrong. either way, it works great now!