So hey! I finally did all the stuff I wanted to do! I had posted a couple times in this thread to talk about putting Izana, Yukimura, Scarlet, and a parentless Rhajat into my Revelations file. I finally got the tools necessary to do so.
And something funny happened. To get Rhajat in my game, I bought a Logbook Rhajat from my Birthright file and then "de-Einherjared" her by removing the white background and green shield. She appears in My Castle, has level-up quotes, all the usual hallmarks of a normal unit. Including supports.
Here's where it gets weird.
In Birthright, she was the daughter of Hayato and Felicia, and married Hisame. Oddly, that transferred to Revelation, and she can have parent-child supports with Hayato and Felicia despite the former being a bachelor and the latter being married to Hinata this time around. But what's really odd is that she also has a textless, C-A support with Hisame. At first I thought it was some kind of glitchy transfer of the fact that she married him in Birthright, but now I realize it's because she's technically his half-sibling in Revelation and the game actually recognizes that... partially. It IS textless supports, after all.