How can I measure outstanding debt? How much "stored value" is in Ripple at any given moment?
You can issue the account_lines command to a Ripple server to find out what credit lines have been extended by or to an account.

For example, you can use this websocket test page to issue websocket queries. Set the location to wss://s1.ripple.com:51233, click use secure, and then connect.

Then paste this in the Message window:
{ "command" : "account_lines", "account" : "rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B" }

This pulls the credit lines that Bitstamp has. You'll see entries like this:

     {
        "account" : "rrpNnNLKrartuEqfJGpqyDwPj1AFPg9vn1",
        "balance" : "-501.64408854616",
        "currency" : "BTC",
        "limit" : "0",
        "limit_peer" : "10000",
        "quality_in" : 0,
        "quality_out" : 0
     },


This means that Bitstamp owes rrpNnNLKrartuEqfJGpqyDwPj1AFPg9vn1 501.64 BTC (Bitcoins) and that user has extended them a credit limit of 10,000 Bitcoins.