I use a DIY version of that, which is to say a key pounch fromy my undergrad years... clip end attaches to a belt look, the pouch goes inside of the front of my pants. Large bills, ATM or credit card, and photocopy of my passport all go there. A pickpocket could cut the belt loop, I suppose, but the strap is pretty firmly wedged by belt and waist and the pouch itself is too big to either miss falling down my leg (if it didn't just get stuck in the drop of my pants) or to get pulled out without my cooperation (would be easier for a thief if not for the manly beer-gut B) :P )

Not big enough for a passport, though. I've always locked that in a room safe (or hotel safety box at older places) and just carried the photocopy.

In-transit, passport goes in a zipper pocket inside my jacket or vest, along with my wallet and cell phone.