I have not used them myself, but several SM friends of mine in the city work through temp agencies. The agencies seem to be pretty good about matching your availability with the needs of the various companies they contract to. For instance, they have been able to say "I'm leaving town for a bit in two months" and come away with either one company that only needs a two month assignment, or a combination of companies that will keep them employed for the majority of the time (a week with one, three weeks with another, etc, etc).
And don't rule out waiting tables, bar tending, barrista jobs, etc. Theatre folk are basically the lifeblood and backbone of the service industry in NYC. Most places are well used to hiring people, seeing them leave for a period of time, and welcoming them back when they return.
