At a non-towered airport in the US, what should I do if I hear a medevac flight waiting to take off while I'm turning base?
The most obvious thing to me would be to just talk to them:


  Medevac 123, this is the Diamond on base for 31, do you need to expedite your departure?


If they say no, then just continue and land as normal. If they say yes, then I'd get out of their way:


  Medevac 123, roger, we'll extend our base and fly a wide upwind to let you  out


I wouldn't worry about standard phraseology too much, the critical point is to make sure you both understand each other and if there's no well-established phrase for what you need then just say whatever seems clearest.

Extending base to upwind has the advantage that it keeps you in the pattern so you can see what's going on as the medevac flight departs and you don't waste too much time going around the pattern again. (Although unless I have an emergency of my own I'm more than happy to make whatever detour is necessary to help out a medevac flight.)

Turning out right puts the pattern behind you and if you turn too early then you're potentially turning into other traffic on a long final, so it wouldn't be my first choice. But I don't think there's any standard way to break off from base and there are lots of possible variables (terrain, traffic, airspace etc.) so it's up to the pilot to decide what's safest at that particular moment.