The AT822 runs off of 1.5V, phantom is 40V. I'm thinking if you make a custom cable for it that has two male xlr at the camera end, and one female xlr and a little battery connector for the mic, you will create smoke. Might as well buy some nicer mics that run phantom.
Also, you can run the AT from rechargeable AA nimh which will last all day (haven't tested it, but it isn't a huge current draw). There isn't any real need to mess around with that.