Are there food safety issues with sous-vide cooking

You could use oil, but I don't think it's worth it; realize that you're going to be using additional energy for obtaining, cleaning, and disposal of it.

Regarding using a broth with a porous bag, well, that isn't sous vide. That's more akin to a slow-cooker. The whole point of sous vide is the airtight barrier between food and heat.

Update

Oil does not have a higher thermal mass than water. Ammonia does, but I'd suggest avoiding that. ;)

Source: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

There's a misunderstanding possibly of what Heston suggested for the British Navy. He suggested to sous vide it and then store it in the freezer. They couldn't just toss it in the cupboard. Sous vide food is not typically shelf stable. It has been pasteurized - not sterilized.

Pasteurization reduces pathogens to a safe level, not to a 0 level. It doesn't typically reduce botulism spores to 0. Once the food is exposed to normal temperatures, any anaerobic pathogens will start to reproduce again.

To sterilize you'll need to pressure cook meat through to 121 C for 2.4 minutes - at which point its rather not tasty. You might be able to sous vide at a lower temp for much longer, but I suspect it'll still be past the point of being tasty. Sterilized meat tastes like your typical potted meat.