The 'raw transaction' RPCs look interesting for that
== Rawtransactions ==
createrawtransaction [{"txid":"id","vout":n},...] {"address":amount,"data":"hex",...} ( locktime )
decoderawtransaction "hexstring"
decodescript "hex"
fundrawtransaction "hexstring" includeWatching
getrawtransaction "txid" ( verbose )
sendrawtransaction "hexstring" ( allowhighfees )
signrawtransaction "hexstring" ( [{"txid":"id","vout":n,"scriptPubKey":"hex","redeemScript":"hex"},...] ["privatekey1",...] sighashtype )