I'm not concerned about running accessories without the engine running. I generally don't do that anyway. And I'm not looking for a full-time replacement, so cold weather isn't a concern either. Really what I am looking for - is a temporary use, super lightweight battery for racing. If safe and reliable, I'll keep my 12# atv battery in the car for daily driving, and then swap in the .5# battery on raceday; drive it to the track, race it, drive it home, then swap back to the 12#er.
I've already gone from the 30# oem battery, down to a AGM 12# battery . . . but why not get really lean on the battery on race-day, especially if you can do it for a relatively low cost - like $40?
My main primary concern; is the small RC l-ion battery safe to use with the car recharge system (no fires, explosions, etc)? From a safety standpoint, if the alternator isn't made to charge these batteries and overcharges it (and there is no safety), then you are looking at the potential of an underhood fire or explosion. Not good alternatives.
It could be as simple as knowing if the Shorai battery has some sort of fusable link or electronic controlling device to prevent overcharging - and whether or not the RC batteries have that same safety . . . but I can't find that information anywhere.
From a reliability standpoint - I'm assuming starting the car once or twice will basically completely drain the battery - so if the recharge system won't recharge it, then i'll be dead-in-the-water and need jumped at the track. This also sounds like an operational/composition comparison with a Shorai battery could give us the answers . . . if I could find that information (which I can't).
As far as water - they use l-ion batteries in radio control boats. So a water-tight battery box from one of those *should keep a battery completely dry (I would hope).