I drink caffeine free Diet Coke daily. It is the one thing I do that goes somewhat against the WOE but so far no problem. IMHO I would try the diet pepsi in limited amounts and watch for trouble. If you think it's slowing your loss the stop. If not just cut down....we're probably better off without it.
Personally I would have more concern with the caffeine as I think it slows my loss. I've all but given up caffeinated drinks.
Have a blast over seas!
Also congrats on the excellent progress so far!