I searched this forum and the API docs about how add custom data to an .xrns file
so that it will be saved together with the song, but I cannot seem to find out how.
I guess custom data could be saved in the song comments, but that does not seem right…
I also looked at the song schemas, but did not come up with anything.
Can it be done?
We don’t have an official ready-made location where custom data can be stored, but it’s still possible to do so if you can find a clever place to stash it 
Exactly what type of data are you trying to save, approximately how big would it need to be, etc. ?
Yes, I though about that and looked at maybe storing it in an unused pattern or device or
something, but it still seems like a hack 
I am writing a script to save pattern locations, so it is not much data I am looking to save,
max. a couple of kilobyte I would guess.
Hehe, that’s it, thanks!