Please explain this in more detail - what does subst achieve? I still see an absolute path, so this would not work across devices if the paths are different, eg because one is a Windows machine (in the office) one is a Mac (in the lab) and one is a linux laptop.
I have created a related feature request here where i point to a successful implementation of this feature in zettlr Link to external pdfs in local Zotero folder and Better BibTex support