I can't say I have a perfect solution for you, I will admit for your situation specifically it's hard. If I were you, I'd just say "This game focuses on bottom content, but there is a good amount of top content in it as well. There are some characters in the game who can only top regardless of the position you picked. There are no plans to add scenes for them to the bottom." on the itch.io page. If you can, maybe list out all the total tops (and total bottoms if you have any) in a document that the itch.io page has a link to. An option that warns which characters are total tops (or total bottoms if any) as soon as you meet them would be nice, but maybe that's too complicated.
I like your suggestion. I'll update the FAQ section of my page and I'll consider making an external spreadsheet or something listing who's a top, bottom or versatile. I still think is kinda spoilery, but if you're warned before going there, I guess it's alright.
The warning as soon as the player meets them... I don't think I'll implement it in my current project. It's kinda like spoiling that this character will have sex with you (I still consider fun to discover that character might have, or not, sex with you. For some characters is pretty clear, for others it isn't), and I'm on a point in the development that is not possible (well, it could be. but doing that for every character would take weeks, and I can't afford that), but it's definitely something to consider when I start my second project, who knows. I'm not closed to that suggestion either.
It's good to have these discussions, there can be solutions for each case, even in complicated ones like mine.