I need to cancel a transaction that was sent without fee (using Blockchain.info). It is currently "Unconfirmed". The only way I have found is to do a double spend of the whole wallet, specifying a higher transaction fee.

However Multibit and probably bitcoin-qt will not allow me to send more than my "balance". How can I bypass this balance check?

Where can I request a miner to include a raw transaction in a block (against a bounty)?

Thanks for your help!