My best guess is that some people wouldn't mind, or even feel flattered, but others most likely would mind (especially if you hit something they're not into at all), so it'd be a bit of a gamble at best. (Personally I've written a few stories featuring others from here/discord, but only with their permission and creative input)

I'm more curious about the practical problems, though. How are you gonna tell a (named) user's gender (save for some very obvious cases)? Much less their appearance/traits or preferences (and more importantly turn-offs/taboos) in regards to guro. Not like there's any sort of user profiles like on other forums.

Maybe just make another post and simply ask for people that would want to star in a story. Less surprising, yes, but you also won't risk offending/creeping out anyone for any reason.