Thanks for the information, everyone. Frank, I certainly am capable of stunning lapses in common sense (details to be withheld), but I think this chained-wallet experiment may not be one of them. The chain will be attached to loops that I found sewn into the pockets of the travel pants I wound up purchasing, so that it will not be visible except when I remove the wallet. I would not carry anything in it beyond the day's spending money (just as I did with the clip wallet on a recent trip), so it will not be the end of the world if I do lose the contents; plastic/passport etc. go in the money belt. If I were to get robbed, I would just go back to using a (spare) clip wallet.
As for the possibility that a thief might extract the wallet and empty it without my noticing, there must be a way to "lock up" the zipper pull so that you have to fuss with it in order to unzip it, as in the Pacsafe products. This will be another one of those little projects to work on in anticipation of the next trip.
And speaking of Pacsafe, I found this: < http://www.amazon.com/dp/B004LLCZMA/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=276FZE0PHDDYQ&coliid=I3475XQ7MQGWSP&psc=1 > This might do the trick, though I would still replace that clunky connector on the end of the chain with a small s-carabiner, and shorten the chain to reduce weight and bulk; also (strangely for Pacsafe) it does not look as though the zipper pull locks, and if so that would need to be tweaked.