I just realized (after last night's turkey-loaf, with a second, very small loaf for my mother, who likes her poultry extremely well done) that the baking times for a single cupcake and a 9x13 rectangle are, of course, vastly different.
Given that this is an emulsion cake (DH Spice, now part of their "Signature" line), so it's not something that's going to fall if you look at it cross-eyed, and that if one of them either falls or burns, I'd much rather it be the cupcake,
1. Start them at the same time, and pull the cupcake as soon as it's done?
2. Let the cupcake sit for 20 minutes, then insert it near the end of the baking time for the big cake?
3. Bake them sequentially?
4. Press the toaster oven into service for the cupcake?
5. Microwave the cupcake in, say, a custard cup?
6. Give up; it was a dumb idea to begin with?
I think I have aluminum foil cupcake cups on hand, and might also have paper ones.