It's not currently possible to do this using the regular exchange APIs, because of the way in the partial signatures in exchange transactions work. As you may know, the last party to the exchange can use completerawexchange to add metadata.
However if you want every exchange participant to sign the metadata, you need to built the exchange differently, using createrawtransaction to create the full transaction representing the exchange, then each relevant user checking and signing that transaction with signrawtransaction before someone (anyone) transmits it with sendrawtransaction.
