Is a multisignature transaction propagated through the Network before it accumulates enough signatures?
Mike Hearn talks in his speech about the future of bitcoin and mentions subchannels when doing micro payments, is this what you are looking after?

When i try this in Testnet
getreceivedbyaddress does show the amount sent to the multisig address, but listunspent does not. So even if listaddressgroupings include the multisig address, you still need to know the txid to be able to spend that prev_output.

if i try to send a tx with only 1 signature, this is what i get in the log:


ThreadRPCServer method=sendrawtransaction
ERROR: CScriptCheck() : 4378d77e5d VerifySignature failed
ERROR: CTxMemPool::accept() : ConnectInputs failed 4378d77e5d


and with 2 signatures (Complete):


ThreadRPCServer method=sendrawtransaction
AddToWallet 470a194f21  new
keypool reserve 9
keypool keep 9
CTxMemPool::accept() : accepted 470a194f21 (poolsz 5)
AddToWallet 470a194f21  
received getdata for: tx 470a194f21


txid is also returned 470a194f21 by the client.

My answer is:

No, the mempool will not work as a subchannel (right now, if it ever was indended)