There's a site that popped up in my Facebook feed called "Funky3dfaces" that makes realistic heads from photos for lego figs, but they look like they could be the right scale for 3.75" figures. Has anyone used the site? It's about $33 per head, but heck, for some really unique ones, that's in the range I'd be willing to pay. I suppose I can email them.

I am not associated with this site in any way, just thought they looked like they could work with the good scale.

Well, I emailed them. Their heads are 17mm tall. That's kinda in the neighbor hood. They also said that they would soon be releasing action figure sized heads, so I guess there's that.