Yes it does! I was misled by the part of the docs that stated, if a component is not in the custom theme, then it's not themeable. Maybe this works because it's a child of a themeable component? You have to be careful about adjusting the import paths in UntypedItemComponent, and to import the component itself, but once you jump through a couple hoops, it works.
I'm still not quite where I want to be -- it seems as if the smallest level of refinement is <ds-generic-item-page-field>, and I need a bit more than that. For instance, we have dc.identifier.issn, isbn, uri, and a couple others. I would like to group them together in the same pseudo "field" (as I might do with field=dc.identifier.*) but still label them by type individually.
There's a frustration level after getting pretty adept at doing these things in xmlui, but with every new tech there's a learning curve!
Anyway, thanks for the tip!
~~Bill