peeks in MAT zones for eyeliner or lipliner might be rough, since it ends up being very different shapes and you'd want to keep naturalistic fades around the edges (which mat zones can't do on their own). One of the biggest kicks in the face to realism is hard MAT zone edges on organic forms; we don't have those on us. ;) If you look at closeups of eyes, lips, etc. on people, you'll see a tiny fade from one region of color to the next. Without that, things start to look more doll-like and less natural. Bear in mind I say all of this as something of a UV nitpicker of doom and someone who lurves a ton of MAT zones to tinker with, too. ;)

You could arguably get a similar effect to what I think you're hoping for with alpha masks and blender nodes for a much broader range of styles, but that would require one of the python scripting geniuses to code something up to swap them around with the way the materials presently work. And you'd want the scripting... I've been picking at a project like this in just the material room to try to arrange something that's versatile and swappable without being too confusing for the end user, and keep tabling it since it's quite tricksy. (It is, ironically, deliberately doll-like. ;) )

I know I would be voting for an eyebrow zone, on a separate plane. Possibly a doubled plane to allow for good visual depth. :) I also have a slight case of 'missing it, could be useful' for the pubic hair plane on V3. Also a fan of the eye surface. Those tend to be handy ones, but they require the geometry to back them up, and some folks seem to keep wanting to kill off those bits of (IMHO quite useful) geometry.

-D
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It's all fun and games until someone loses an eye texture.