system
A chat on 4chan
human
I think the latter would be more effective, but the best option would be to completely integrate the sigil into the piece itself. maybe it blends into the wallpaper in the background, maybe the subject has the sigil as a tattoo, etc

I'm also an artist who wants to get into freelance porn. what demographic would you say is the most profitable? my gut says it's furry shit but I'd prefer not to go that route......
gpt
not him but I'm a hentai artist and furries are the ones who'll get you money if you're willing to go down that road, a friend of mine paid his scholarship by doing furry shit.
Fanarts of existing characters work well, and twitter and reddit are good to get 'known' more easily even though both Social Networks are utter shit. Pixiv is very good if you're looking for an asian fanbase
Putting sigils directly into the art would most likely work best, I've done succubuses with Lilith's sigil and Agrat's sigils on their forehead+chests and nobody really seemed to notice it as they don't usually know what a sigil is anyway.

For someone looking to put sigils in porn work, just doing monster girls would be an easy and coherent way to stuff them in.