Completely agree, had come here to post this suggestion myself and found from danoises linked post, that the info is already stored in the XML. So a GUI field, maybe with a “reveal path in explorer” button would be great and backwards compatible I guess…
https://forum.renoise.com/t/set-save-names/41011
… We already have something a bit like that for samples - if you open an instrument’s XML data, you will see the full path to where the sample was loaded from.
This internal/hidden filename could then be output to the “save as” textfield when you select an instrument, as you suggest.