You should take a look at this page:

https://www.posersoftware.com/documentation/13/index.htm?t=Poser_Reference_Manual%2FOnline_Cover_Page.htm#t=Poser_Reference_Manual%2FOtherApps%2FPoserPython%2FPoser_Python_Addons.htm&rhsearch=erc

Even though it's for the most recent Poser version, some Python classes exist since V9 if not sooner. One of them let any script do something at file saving time.
Maybe it's not related to your problem, but when all "standard" ideas have been checked and nothing found, the response could hide somewhere else.

𝒫𝒽𝓎𝓁

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