When you edit a page template, you can apply a filter to a frame so that it only shows selected flows.
Note that this filter is not available when you’re editing a frame directly on a page. So you’d probably want to create a new page template that contains one short music frame at the top, filtered to just the one short flow, and a larger frame underneath, filtered to your main flow, and then insert a page template change. (Actually, you’re probably going want to edit your existing templates as well to filter the music frames to the main flow – otherwise, they will display the short flow with the duplicated material after the main flow.)
Make sure that the frame for the main flow is part of the default MA frame chain, and the frame for the short flow is some other frame chain.
Finally, since you can’t set first system indents separately for different flows, you’ll need to remove any indent for this layout (in Layout Options > Staves and Systems > Staff Labels > Indents) and then manually re-apply indents to other flows with the note spacing tool.
Final result:
Repeated system.dorico (761.8 KB)
On the whole, @Janus’s idea of a graphic slice is probably easier, though the quality might be slightly less good.