I've always used a backlight with greenscreens to wash out green spill. It also provides a bit of a rim/hair light. Now I need the same thing, but I don't want to have any backlight, I want it flat and dull and not flattering. Is there a way to accomplish this with a gel and still get some of the benefits of washing out spill? I don't want that backlit look.

Try using a minus green gel on the backlight. It will kill the green spill without having to be as intense as a non-gelled backlight.

Thanks, presumably I'd want the strongest 3/4 step minus green?
Gam 1/2 Minus Green Cine Filter (20x24" Sheet) GC1582 B&H

Use Primatte in After Effects.

I've never used a backlight and can key stray hair.