Nightshade or Rustoleum lens tint, no sanding. Just like you said, totally reversible with just gasoline and cloth. You mistake, you remove, you do again.
I like the idea and added protection of tint, but out is quite complicated. I'm sure that someone has done it correctly, but most anytime I've heard of people trying it it hasn't gone well.
FocusFest 2013: Best of Show
Gateway FFW: Show Class Outstanding
September '13 ROTM