I have the same problem- except it's a left turn. I go around a major corner on a busy road on a big downhill- then I have to turn left. I take the lane, and I therefore can't slow down too much (don't want to hold up traffic any worse than I already am). For me, the need to keep both hands on the brakes is as much about balance as anything else (and in my case I'd be signalling with my left hand). Usually, I don't feel comfortable taking my hand off to signal (and I guess I kind of assume they'll figure out that if I'm just to the right of the yellow line, that I'm out there to turn). Sometimes I'll point with a finger (more or less keeping my hand on the drops), but I'm not sure if anyone actually sees that.