I booked a room twice on the spot last week - first I decided to extend my trip by an extra night, then I went back to my room to change the flight and realized flights were cancelled anyway due to a storm back home, and had to go back to the front desk and add a second additional night. I'm pretty sure they thought I was crazy until I explained it.
When you book it you'll get a confirmation number which you should be able to use to just walk up to the ME counter. I've gotten on the ME twice just by quoting my parents room number (they'd already checked in and added me to the reservation at check-in) and once with just a confirmation number scrawled on a post-it.