Boy, I hate to be a naysayer but I don't think just using co2 in over a gallon of headspace is a good idea, even if you did have a co2 tank.
The reason is that while co2 is protective, it doesn't hang around indefinitely and once you add it, it will immediately start disipating to reach equilibrium. Since meads are in the fermenter a LONG time, you'd have to do it often and even then I'm not sure it would stay for as long as you'd hope.
It'd be better to rack to a 3 gallon carboy and a 1 gallon jug, and top up to the bung so that any risk of oxidation is minimized. Using sulfites is also a good idea, as it will bind to the mead so that oxygen can't.
I have a kegging setup, but I still top up all of my wines, meads and ciders.
Another issue will be degassing when you want to bottle the mead. Much of it will off-gass, but you may risk having a gassy mead if you continue to flood with co2 and keep it airlocked.