Thanks Kevin!
This is what confused me about the Resolve manual’s explanation of this feature. I knew that this was supposed to be possible in the Format tab, but it never occurred to me to use the PTZR controls on my control-surface to pan the still! I had been dragging the sliders in both the “Input” and “Output” sub-tabs, and was totally confused that only the main source clip would get panned, and not the still.
It seems rather counterintuitive that the reference repositioning feature seems to be completely inaccessible via the Resolve UI and only works via the control surface. But I’m glad to learn that it works, in any case.
Now, if I could only rotate the split, then I’d be even more happy!