Here's the source photo and a crop of one bloom.
Foliage, stems, growth habit, etc., can be a big part of identifying flowers, and even though I wasn't focusing on including good shots of the leaves and stems when I took the photo, I think there's enough showing to be some help.
Thanks to those of you who have already weighed in, and any who might still choose to offer an opinion. Since I had the advantage of seeing the plant 'live', I feel fairly confident in ruling out geranium or cranesbill as a possible ID.
