Don't worry about the expullsion that much. I find that people who have issues with their bum being "Dirty" don't enjoy enemas as much as those who can get past the poop part of enema play, the more you can un-teach this closed minded societal norem, the more you will enjoy anal play in general. My g/f used to have major issues with anything anal, especially expulsions of enemas, but over a period of time she has learned to really enjoy her ass as an errogenous zone.
Everyone poops, don't sweat it- the humilitation and lack of privacy can even be used to heighten the energy of the scene, or bring your submission to a new level. Your taking an enema to get clean, and eventually you will be. In the bible for enemas, Intimate Invasions, author Mr Strict states that the first expulsion can be in private, but everytime after that, it is up to the top to decide.
Personally, while I am not into scat play, I feel that the expulsion is part of the enema experience, and for many women, their favorite part. I often invoke forced masterbation during expulsion, and many women can get intense orgasms from double-clicking the mouse while on the john. Finally, in books and videos, I feel expulsion is similar to the enema money-shot, where one can tell if the scene was bullshit, or if the enemas given were in fact real.