Not very difficult to do (as that's literally what the incest, bestiality, and underage/loli/shota patches do and Patreon at least doesn't seem to enforce any sort of consistent policy regarding it) from a programming perspective. However, the game itself would probably be a bit wonky storywise as you'd be adding in large amounts of content that the game doesn't readily hint at. If it readily hints at the content from the patch, it is much harder to play off the "fan made" patch pretense. This makes any sort of story driven game a lot more difficult to write for unless the base game is just a constant amounts of fade to black and afterwards scenes, which again leads it to being very obvious the fan patch isn't done by fans.
Plus, like the example above, there are some people who have been creators of porn stuff (games, videos, art, etc) that have been doxxed. So it comes down to if you want to take the risk.
So Patreon probably wouldn't care less, but law enforcement could be an issue.