I was planning to overwinter these indoors in a plastic bag with barely moist vermiculite, as with Achimenes rhizomes. Do I risk having them dry out fatally? Is there a better way to store them?

I keep Seemannia (and Achimenes) rhizomes bone-dry over the winter, usually stored in a small amount of dry potting mix. As long as you keep them cool and dry they will stay dormant for several months. If you give them any moisture at all, you risk (a) rot or mold and (b) premature sprouting. They will eventually start sprouting regardless of what you do, but even then they will be okay for a few more weeks. I usually start them under lights indoors in early spring and put them out once the weather has warmed up a bit (usually early May).

Here are a couple of photos of S. nematanthodes growing in a couple of different places:

I wound up getting this plant after seeing photos of it in your garden - it's a beauty.

I've grown other gesneriads in the border, including Sinningia sellovii, which is a vigorous plant that flowers much of the outdoor season. S. tubiflora has been good in a tub. This past summer I tried S. "Towering Inferno" for the first time and it did OK - maybe with a bigger tuber it'll be more decorative in future seasons (some of my S. sellovii tubers are approaching potato-size).