Perhaps do a little thinking about who your customers are. Just guessing, but I would think they are people with a) not a immediately threatening problem, but one they want to see a doc about, b) have $25 or a credit card or something and are willing to use it to prevent waiting around, and c) are internet savvy.

So I'd go looking for a hospital that has a lot of these folks that use their ER and see if you can't set up a partnership based on the work you are currently doing with this hospital. Better yet, get a list of 50 such hospitals and work your way down :) With a high-enough density target market, you can move to things like radio, doorhangers, or newspaper ads. The interesting question is whether or not to use SEO, because of the geographical limitations.

Neat idea. I was just talking with a guy a couple of weeks ago about this exact thing, except for doctor's offices.