The code in the achievements zipfile here uses them. I’ve attached a compiled version of achdemo.inf.
When I was last working on the project, I converted achdemo.z5 with zcode2js.py and tried using it with a local I7-generated parchment template, but the “save files” appeared to be encoded as urls and it didn’t seem to distinguish between a normal save and a save to an auxiliary file. After testing with various interpreters, I concluded that glulx aux files were more or less working, but that using z5 aux files in the way that I wanted would require interpreter work.
achdemo.z5 (103 KB)