If you need a long cage to wrap chain (such as for a double in front), you might try a Le Cyclo clamp-on. Clamps on to the chainstay. Cyclo mechs usually have nice long cages and a ton of side-to-side adjustment via the central pin or axle on which the derailleur slides laterally. Disadvantage is that you can't remove the back wheel as easily.

They can usually be found for affordable prices on ebay France, if you look hard enough. I got one for under $100 all-up, just this past summer, but I had to buy the clamp separately. I think the derailleur/shifter/cable/housing was $50 and the clamp-on mount was another $20.

The "Oppy" even has its own spring so you don't need the long spring running under the stay.