Very true Traci, but as Lyne said, let the end user worry about it. You can't give end users every possible morph for the hat so they could use whatever hair they want. Hair products are so different, there's no way to know for sure what will work well, and what won't. I think you're trying to do the impossible.Even that isn't going to help much because the morphs are so unique to this hat "fit." A hat morph is not likely to morph the hair the way this hat fits - you're more likely to see a morph for a hat like the Cowboy hat which sits on the top of the head and not "around" the head like this helmet does.
The only thing I can think of, and at this point I'm not sure you'd want to deal with it, but actually modelling some hair bits at the neckline and across the top in front as part of the hat, is the only way you can be sure folks won't need to worry about what hair to use. Years ago there was a product like that, I think for V3, IIRC that had that.
OK, I just checked, and I was wrong, the hair was separate, but made to fit under hats from another product ... just the hats. Not sure why I thought they were one piece.