There isn't currently a way to transmit it between nodes without embedding it in a blockchain transaction. If you don't mind the exchange offer being visible to all nodes and embedded in the blockchain, you can use this general method for embedding data in a chain:
http://www.multichain.com/developers/creating-data-streams/
The next alpha of MultiChain will have a far more scalable wallet that makes this perform fine, even with a very large number of transactions.