Thanks, @solver. Yes, I am able to use that "Frieze 7" just fine. Thanks for that!
I have a bunch of the bonus catalogs. From what I've seen, the only one that includes any railing is the AZEK Building Products one. I did get 3 railings with that. (6 profiles, technically. An upper rail and a lower rail for each set.) But the support for this in Home Designer seems like almost an afterthought -- or maybe even an accident. I sort of stumbled upon it. I don't see a tool anywhere for applying profiles specifically, but I discovered that if I select a profile in the Library, I get a cursor that allows me to apply it to areas that support it. (Railings, Moldings, etc.)
A few hours ago, however, I had a sudden stroke of genius -- I decided to download the free trial of Chief Architect Premier X9, with the suspicion that I might be able to dig around and find the Core Catalog files it installs. It doesn't let me save a .plan or export my User Library, but I found the .calib files it installs. So I grabbed a bunch of the profiles from there and imported them into my Home Designer library. It didn't let me install everything -- for some reason, "Round Rail" doesn't import, for example, but "Semi Circle Rail," among others, works just fine.
The "Round Rail" profile from the Chief Architect core catalog is basically what I was looking for originally. Even though it's not letting me install that one, I'm pretty happy with "Semi Circle Rail" for my particular need at the moment.