Yeah I didn't think of the deniability aspect.  Those are good points.  I will look into blockchain.info.  If they have API support for importing a private key that might work.  I guess I could also import the private key directly to MtGox.  Just need to check how the confirmations can be tracked. 

If it proves popular security could be enhanced by using automated SMS to collect half of the private key.  Webform asks for the first x digits of the private key and then displays something like "Text the second half of the private key to 11011".  SMS gateway service could relay that to the backend server which combines the key.

Anyways just to be honest up front this isn't a priority right now but it is an interesting idea and will help to improve liquidity so it is something we will implement when time is available.