It wouldn't be a terribly difficult script, although there is one key limitation that would complicate things.  PoserPython wouldn't be able to create a new group for the hairs it would add.  So it would either have to work from a user-defined symmetrical hair group in the OBJ or it would have to append the new hairs to the same group used by the side which is being mirrored.

I'd kind of hoped someone would jump in and say there is a pre-existing tool or script.  If there isn't, I'll try to tinker with the idea and see if I can develop anything for Python.

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.