Sous Vide and Plastic Melting

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

First of all, I agree with the others that there is no harm done by plastic bags for sous vide. I have read a statement by the manufacturer that brand-name Ziploc bags don't release anything below 76°C. If you think how much a lawsuit could cost them if the information turned out to be wrong, I trust that they are telling the truth. For other brands, you may have to do some research about safe temperatures.

If you are still unconvinced, your choice of material is very limited. You say it must be airtight and waterproof; I'll add that it must be pliable, so it can cover an irregular steak shape perfectly, without leaving air pockets, and it must somehow be able to create a seal. Also, it must not release any harmful chemicals by itself.

About the only thing that fulfills all criteria would be a wax with a high melting point, like carnauba wax. You could paint the steak with the melted wax, or, probably better, you could soak a piece of gauze in the melted wax and wrap the steak in it, pressing out any air bubbles. Then let it cool and set before cooking. The downside: not only is the food grade wax difficult to source for private people, it also can end up costing quite a bit.

Another option would be silicone, like the one used for baking pans. Unlike plastic, food grade silicone does not contain any chemicals which could leak - the medical sector uses the same stuff for prostethics and implants; it is safe to have it inside your body, so it is definitely safe to cook your food in it. The problem is that you'll need a sealable bag made of silicone, and I don't know if anybody manufactures such bags.

You might consider very tightly wrapping the meat in a caul (not necessarily an amniotic caul, a peritoneum should do nicely) and binding it, but I don't know where you can get cauls. Maybe you can ask a butcher. Also, it probably won't be 100% watertight (but still enough to keep the tasty juices in the steak where they belong, instead of having them flow out into the water).

If you are willing to relax your rules a bit, you can solve the problem much easier. I think you will agree that whatever hypothetical substance might leak from a plastic bag, it cannot travel far through a solid medium. So a steak wrapped in something protective and then sealed in a plastic bag should be safe - you just discard both the plastic bag and the presumably contaminated protective substance. Yes, it is possible that meat juices that have come in contact with the plastic bag end up on the steak, but the possible contamination should be hundreds of times less than if direct contact is allowed between the plastic and the meat.

If you can live with this option, the usual insulators used in the kitchen should do. They are plant leaves - I'd use grape leaves, but you can use practically anything that is big enough - and batters, like tempura batter (breading leaves an irregularly shaped surface, so I won't consider it here, you'll end up with lots of miniature air pockets). If using a batter, you will want to first set it in a pan with very hot oil, just hold it there long enough to set the batter, but not enough to warm the meat on the inside above the sous vide temperature. Both of these options wont't function on their own, as they won't create a sealed waterproof barrier. But combined with a plastic bag and later discarded, they should be a good solution. The leaves will also add a nice taste of their own.