It's not a pipe dream, but the answer is going to be a combination of technique, setup and perception.

There isn't that much volume added when you bolt a resonator to a banjo, it just seems that way because it makes the sound so directional. If you current banjo has any kind of a tone ring you can boost the volume pretty simply without adding a resonator.

Old time banjo purists might want to stop reading right about here and avoid a heart attack.

Pick up a plain-jane top-frosted Remo head and tighten it until the feet of your bridge don't skin into the head anymore. Don't try to tap-tune it to a note, just get it tight.

Lower your action.
Pick up a lightweight bridge and a set of lite-gauge strings (I really like the GHS JD Crowe set). My personal favorite when it comes to bridges is either the Shubb or the Grover compensated. Moon bridges are pretty good, but the stair-step design of the Grover & Shubb bridges work a little bit better when it comes to keeping individual strings from bleeding into each other. The other advantage to the Grover bridge is that it's cheap. Something that's cheap and works well is always a plus in folk music.

The last thing I'd do is pick up a heavy adjustable tailpiece. Price or Kershner is the best made today for this. You can pick up a lighter licensed copy of the Price for a few dollars less and it'll do the same job. Whatever tailpiece you use, crank it down until it's just shy of the head. That will put tension of the lite strings so you wind up with the drive of heavier strings but keep the playability of lite gauge.

I've used this setup on quite a few banjos ranging from the Wildwood I play every day to the Gold Tone Cripple Creek banjos we use as loaner instruments in our workshops. The results are almost always the same in terms of volume.

-Patrick
http://patrickcostello.blogsome.com