Thanks, all!
John, I accidentally underexposed by just over 2 stops. (One irritation of the Canon 5Ds, and possibly other cameras as well, is that you can't say "underexpose this shot but then revert to normal", or at least I've not found out how to do that; my absentmindedness results in the next shot (at least) being wrongly exposed. But I digress). I thought I could do something with the result anyway. I went out that evening to take some more, similar shots, but didn't return with anything I liked very much.
Jeremy