What we need: a graph showing the number of transactions to and from 2-of-3 multisig addresses plotted against time.
I'm working on this at the moment. Parsing the blockchain is taking forever.. Took an hour to get to 83000. Then a further 19 hours to get to 128500.. I'm recording txid/vout/address/blocktime for all payments to scripthash addresses (version byte \x05).
Once I have this I should get a nice plot of payments to multisig addresses, and then rescan looking for vins spent from multisig addresses. There'll probably be a good deal of lost BTC sitting at these addresses.