I gave your replies a shot, and those solutions just won't work for me. In addition to being cumbersome to implement, they don't allow me to 'build up' my paint by going over the same area later with the same brush. I suppose some kind of spray tool that could continuously spray the same brush at a low setting would work, but I can't find any kind of plugin that allows me to do this. Thanks for the replies.
edit: I suppose I will ask here that this feature to be implemented. The ability to paint with smooth, opaque brushes allows you to do all sorts of nifty things, from water color effects to realistic color blending on your canvas. I think if paint.net had this feature it would be the perfect program for my purposes, and probably for many other people as well.