On the good news the AIP plastic is fairly sofy and "carvable". . You can remove the kepi with the neck scarf, clean up the collar area then add aonther head. Unfortunately that requires sacrificing another figure just to get a head. There are some Historex heads that might work in this scale.
For the French officer in Kepi I used a TSSD ACW head (for t he hearded face) in a UP kepi. I cut off the US hat, created a level surface for the new hat. I glued down a strip of styrene that would be the bill of the new kepi, after everything was solid I built up the hat with two=part epoxy putty (Kneadatite). A little fiddling and smothing and voila - a reasonable kepi.
I don't know enough about French WW1 uniforms (yet) but I imagine that some of the colonial troops may have had disctictive head gear before every started wearing helmets. That can also add variety. One could use French heads with Adrian helmets, add them to Doughboys, change the rifles to French Lebels and have a representation of the black US troops of the 92nd Division.
Gary B.