Have you see the work I did with the solid particle system for doing custom sprites?
Sprite character controller
I honestly really really like using the Solid Particle System for rendering sprites. With some tricky setup you can have different systems for different sets of particles that are all very customizable and controllable.
You can do some of the most amazing things with it, render planets and asteroids, do blaster effects, rain, sprites, trees, birds, etc. Its really one of my favorite systems to work with. Cant off the top of my head remember the original author but they killed it with this feature.