smoothness 

as long as whatever derailleur you have matches the same curvature on the blade side as a "normal" half-step derailleur, you're fine. you could probably just use anything that's meant to shift a triple down to the minimum chainring that you are using.

addendum:

this is probably not what you want to hear, but from someone who supposedly had the quads to deal with a half-step gearing: it sucked. it might have been because i put it on a mixte with swept back bars, but i never felt like i was in the right gear, and riding uphill was bad. this is when i was young, and still had quads. this was a complete deerhead set up that i lifted from one bike, and put it onto another one. not sure if it was a special version or not; pretty sure it was the "plain" version. ymmv.