I can't imagine this is in many people's top 10 things to do before they die, but who am I to judge?
Basically, anything which goes into someone's colon and then into someone else's mouth carries a certain amount of risk. Having said that, it's probably not as bad as people imagine - there are plenty of studies which show that communal food gets contaminated with e. coli & other bugs from all those dirty bastards who don't wash their hands after taking a shit.
To put it into perspective, an enema is often used to prepare the bowel for a sigmoidoscopy, which goes up a short way to look for polyps/cancer etc. It isn't enough for a full cleanse, which usually relies on something more powerful - and unpleasant. So, if you're relying on an enema to clean the lower bowel, it'll probably do the trick but people respond differently depending on their bowel habit.
Regardless, you will end up with shit on your face/mouth, just not visible shit. As longs as you take a good shower, it'll wash off! Probably best not to swallow though.
I'd have thought you'd need to have a very good relationship with your WG of choice though and you might not want her to have had a curry and 6 pints the night before!
This is a complete assumption on my part, and hopefully won't offend, but it sounds like the sort of hardcore stuff you're more likely to find talked about on the more extreme gay websites. That's just based on the conversations of the few gay friends I have, whose boundaries are way more exotic than mine. You could perhaps try looking at a few of those.
Anyway, good luck on your quest for sexual fulfilment, this sort of stuff makes me realise just how boring I am!