For me, one thing I hate is a partially painted frame. I've run across numerous cars (and even attempted it myself) wherein the rustproofing was stripped off and the bottom of the car painted black. To me it looks terrible because there are so many nooks/crannies that the average Joe doesn't do a good job (or doesn't finish it once he realizes what he got himself into).

I realize your question is a little different, though. I'd suggest a good wire brushing and spraycans of flat black. For some reason flat paint sticks and covers better than other finishes -- and you won't be able to get a shiny subframe without LOTS of prepwork.

I've had mixed results with POR-15 and cheap-quality spraycan - my recommendation is Krylon or Rustoleum in a spraycan.

If you can, preserve some of the details items (shockabsorber washers, brake lines, etc) so that it doesn't look like a guy just washed everything down with paint. A rag dipped in paint thinner works well, just stay away from your fresh body paint!