that's why i use my "tea-bag" method fro brewing in the apartment. i just dump the grains into a garbage bag in my small garbage can. you can dump from the MLT, but you still have to spray the grains out somewhere, and you don't want those going down the drain in your shower. you could spray it out, fill with some water and dump it down your toilet a couple of times, i suppose, until most of it is gone. i always dump my trub down the toilet.
a friend of mine puts them down the garbage disposal. i tried that once, i think i have a crappier, older GD than his...i thought it was going to die on me.
no matter what, the sooner you spray it out after mashing, the easier the cleanup will be. don't let grains sit in ANYTHING for an extended period of time.