Dang... And I was sooo looking forward to the amusement.
Seriously though, what seems to work the best is if you are alone, just go... If it's an extended away, simply log out. Then when you aren't so busy with RL, log back in. If you are with a group of people, I've used the excuse that I need to go use the sewers or something similar, then run around a corner, face a wall, and go do what ever I need to do. Sometimes you'll see people yawn indicating they are about ready to leave either for the night or to go do something else. There are always ways to IC'ly let someone know you aren't there. (I actually rp most tells as a form of telepathy unless the tells are in brackets so using /away as an answering machine wouldn't work anyway.)
*Edig grins, then looks at the fun spoilers* so there... :p
Oh.. and to answer the original question, there isn't a command such as /idle as far as I know... you can see the list of commands by going to the system tab and typing the / followed by the tab key. this will show you what's available. Enjoy!!!