I painted the undersides of my arches white gloss (high traffic area). Made a nice contrast/look.
One thing I've considered is choosing a curved colonial molding to go around them. It can be done, esp. with mdf or foam. Our house is on the water, and it's a colonial, so it would fit.
But man, it's got to be so much easier to just leave em and enjoy the 'open space'.
Stencil idea is neat too.