Good to see you again, Supple. I've missed our early morning chats.
Going with the theme of one of your interests, yoga here are a few ideas.
Being made to receive and hold a coffee enema while in the downward dog (Mukhasana) pose. That might be a bit difficult but isn't very imaginative, I'm afraid. Receiving an enema while in the child (Garbhasana) pose might not be too bad.
For the humiliation factor, being forced to expel into a basin or bucket in front of the dominant partner while maintaining the chair (Utkatasana) pose.
I wonder how easy it would be to receive an enema while in the reclining spinal twist (Jathara Parivrttanasana) pose. That pose might be helpful in getting the water to really move around the colon to give a good clean out.
Holding while in cobra (Bhujangasana) position may be difficult. I held the position (sans water) for a few minutes today not long after eating and it was a little uncomfortable. I'm sure a rumbling/cramping tummy would make it even more so.
Lots of potential with the bridge (Setu Bandhasana) pose or Inclined Plane (Purvottnasana) pose. For example, being made to hold the position while wearing a butt plug, placing something somewhat painful or unpleasant between the pelvis and feet so a higher reach would be desired by the submissive. Having the submissive hold the bridge pose after they have been spanked soundly so their glutes feel the burn of the stretch and the burn of their tush.
Receiving or retaining enemas while in half (Ardha Sarvangasana) or full shoulder stand (Sarvargasana) might prove difficult.
Maintaining the Bound Angle (Baddha Konasana) pose while wearing a butt plug.
Anyway, that's all I can come up with now. Maybe my devious mind will come up sometime later.
~ Dianne