It would be great to have a tag which allows you to spawn / not spawn entities based upon whether the sky is visible from that block (e.g., mobtype:monster & sky:visible).

Have you tried light level tag with skylight option?

I was trying to prevent mobs spawning on the surface. So, I tried light:0-15,sky, but it wasn't clear to me whether that did the same thing. At night, the surface has light level 0 from the sky, and a cave also has light level 0 from the sky, so I didn't think that rule would implement what I want. I do want mobs to spawn in caves... just not on the surface.

@andrewminer sky light level is always 15 on the surface, even at night. use light:1-15,sky to prevent surface mob spawns.

Ah... I didn't realize that, @InsomniaKitten. Thanks, that will do what I was looking for.