I'm thinking about this as well, and from what I've gathered is:
-Its ok to do with a stock-type battery so long as the battery is in a sealed box with vent hoses.
-Its a bit easier to do with a drycell battery as you only need a tie-down, or a tray to hold it in place.
According to my local SCCA solo II region these hold true in autocrossing as well as long as its not in the cabin area of the vehicle (not sure how that works with the hatch).
I think I'm going to get a cheap tray off of ebay ($20 or so) and the drycell battery ($93), and mount it in the trunk. Just need to find a good safe spot for it. This of course is when my stocker dies (its the original battery with 65k miles and 3 years on it, I'm assuming shortly). Oh, and for wiring, just get 0 gauge or 2 gauge wire, and run the hot one up front, and ground it in the rear.
Are the possible gasses lighter or heavier than air? Any possibility of mounting below the tire in the sub spot and possibly drilling for vents there? I admit I don't want to allow too much water inside, as it gets and stays wet here in the winter for months at time. Time to dig around in the back....