I was using RAW, so the resulting photo was very easy to manipulate. In this instance I dropped the exposure of the background to get a more dramatic sky and ground, then bumped up the exposure for the flash-lit foreground. The two images were easily merged: basically I just pasted zumi and the rock onto the other image, and masked out the edges. The grass caused some irritation as it crossed between the light and dark areas, but some creative masking sorted that out.
I tend to forget I have a working on-camera flash now, and I was halfway through dragging my SB-800 out of the bag, perched on a steep section of cliff as I was, when I thought "Fuck me, I've got a flash right here!" Much easier!