Not that I'm against urination content (at least if it's male on female, don't really have a thing for being pissed on myself) it's hard to see from the current characters who would enjoy it. MC is slowly waking up to a dominant side but I don't think he's there yet. Astrid is more into pain and being ordered around, not really into humiliation. Celest maybe since she seems to be more a general sub and hints at wanting to be used and so humiliation might fit.
If the Black Hand chick (forgot her name at the moment) ever redeems herself enough, possibly by doing something insanely selfless I could see it from a penance angle but at that point I don't see MC going for humiliation as a punishment since he is very much of the forgive and forget school.