Are my funds going to be locked in limbo forever?
There is no reason other clients wouldn't relay your transaction so the miners should have gotten it. There's no reason a miner wouldn't include your transaction other than since it has no fee it isn't the first to get included.
If you had sent the payment using Bitcoin-Qt/bitcoind it would get re-broadcast periodically (e.g., hourly or more) until it gets included in a block. However, there is no guarantee that this is happening from Blockchain.info/wallet and it appears that is the client you had used. With your transaction now aged more than a full day, it is possible that it simply is no longer being re-broadcast and relayed and the mining pools no longer even know of your transaction.
If Blockchain.info/wallet eventually drops your transaction (and all your spend transactions since that used those funds from your 6.5 BTC coin) then your funds will re-appear and you can send the funds once again. But there's also a chance a miner does include your transaction and it will confirm at some point ... like in the next 24 hours.
There's no way to add a fee after a transaction has been sent. An approach to allowing that (child pays for parent) is something that has been addressed and had already been coded (pull #1647) but is not part of the Bitcoin-Qt/bitcoind client today and is not used by many (or any?) miners.
So a workaround today ... without any guarantee of it fixing your problem, is to just import the private key for your 13BXjmv6PCP361wKda63KfqH2kkCgv3Z8B adddress into Bitcoin-Qt and try to spend the funds from there. Another approach is to "redeem" (import the private key) into Mt. Gox and let it try to sweep the funds to your Mt. Gox account. Or wait another few hours or a day and see what happens with the transaction (i.e, either confirms or disappears).
