This could also be done with the sender using two wallets (two different instances of the bitcoin app). Wallet1 is the receiving wallet from all sources with a number of different receiving addresses. You then send the aggregate amount to a single address in Wallet2. Then you send from Wallet2 to pay with a known sending address. The only catch being that you need to do the transfer sufficiently ahead of time to have enough verifications in the block chain for the coins in Wallet2.

Your example would change to: