I'm going to play with a concept guys used to refinish Fuch wheels back when I was into old Porsches. (I guess they probably still do it...)
Just fill the well with paint and let it flow around the bottom, then tilt/turn to get it up on the walls. Any overage on to the top surface can be easily removed with some thinner or prep spray.
Or I just go the quick route, do some quick rough masking, spray the whole thing, then remove the white from the top surface.
The "fill the well" is exactly the technique I used on models and when I did golf club refinishing (paintfill). The key is to remove the excess paint about 1/2 way thru it's drying process.