Thank you all. That’s what I thought - no way to make the query the way I was thinking of.
I realize that, if i’d like to have a program search for repos not configured in sources.list, then I would have to specify the repo as well (the https:// link, I mean).
The website half-fixes the problem, since it tells me the release the package might be in. But still doesn’t tell me whether it is in main, contrib or non-free (section?).
Don’t worry about me creating a frankendebian by mixing releases - I destroy my system in other more subtle ways like adding appimage applications as if there was no tomorrow.