I agree - trying to loose any weight at this stage probably isn't a great idea. You need to be as strong and healthy as poss.

You could always splash out on a triple but might cost a few pennies,as you can't just add on another ring. You've got to get an extra long rear mech cage and get your LBS to fit it.

Have you considered a compact chain set? Only 2 rings but the big ring is a few teeth smaller than a standard one which is kind of a half way point rather than going the whole triple, and you don't need to mess with the rear mech.

OR.... you could just relax, exercise light and rest plenty during the last week and I'm sure you'll have a fantastic time. If you do find you have too much equipment you could always pack it up and post it home!