The group editor allows you to set a URL for each group. All pages in the specified directory belong to that group. I don’t believe you can have more than one group with the same group URL. So pages in /orphan-page cannot be in the same group as /office/orphan-page.
Groups can inherit the navigation from other groups though. So you can make it look like a page is part of another group. In the group editor, use the option “Inherit main navigation from” under “Other group settings”.
So you may just need to create a group for /orphan-page and inherit the navigation from whatever group owns /office/orphan-page.
Thanks for the context and suggestions Jon. Seems like I won’t get what I was hoping for, but giving it its own group is the fallback I was considering.
Seems “inherit main navigation from” doesn’t affect breadcrumbs (at least defined for us; I didn’t look too close…), but I see instances of specific edits to the breadcrumbs that I could try to emulate as needed.