Hey Mika, hadn't thought of the marine battery option. Sounds like it could be a good option. As for the CAMS regs, yeah I can see why scrutineers might give an internally mounted dry cell battery the thumbs down - there is no distinction between different types of batteries in the regs. But thanks for the heads up!
And Max, cheers for the call about the heat issue. My thought was to have the battery mounted inside a plastic leak-proof battery box. I was thinking of insulating it. I'd read somewhere that AGM/dry cells don't take kindly to too much heat.