So this topic has come up before, but in broader contexts. For obvious reasons, the BDSM/Kink community has faced these issues for some time - you cannot, in polite society, generally wear a shirt that says “I like doing kinky things” (I have a few kink shirts, ranging from funny to crude, but I basically don't wear them publicly).
There are two widely recognized symbols of the “kink” community: the triskelion medallion and the ring of o - both of which actually originate from The Story of O. The medallion is more recognizable, and looks like this:
https://upload.wikimedia.org/wikipedia/commons/thumb/8/80/BDSM_logo.svg/2048px-BDSM_logo.svg.png
while the ring of O is usually rather specific to practitioners of erotic slavery and lifestyle bondage:
https://upload.wikimedia.org/wikipedia/commons/5/50/Ring_der_o.jpg
While the latter is, thus, likely unsuitable, the former is rather widely recognized as a kink-community symbol, and seeing that symbol likely identifies it's wearer as a kinkster of some sort - yet for those who don't know it's meaning, it's just a simple … pretty design.
I know some choose to add other symbols along with these as well - but this might be a reasonable starting point. It's obviously not specific to enemas, but in my experience, most kinksters (at least the ones that are worth talking to) are pretty excited to talk kink with anyone, even if they don't share a specific kink. Even if another, more specific pin is suggested, also wearing the triskelion would be a good way to start and introduce more people to the design of the specific pin 😉