Basically, I think you would need to change every g at the beginning of a line into an m and remove all the lines that originally started with an m (if Poser produces any of those). Then on the way back you'd change the m's back into g's. I can easily do that in emacs (or use sed), but I'm not sure if the average user would know how to do it in a text editor. UVMapper might be a better solution if user action is required.

I have a pair of Python scripts for the conversion from back when I was working on Antonia's base mesh because I needed to preserve the materials and smoothing zones as well and also do left-right adjustment of mirrored groups.

-- I'm not mad at you, just Westphalian.