I just bought this baking kit from packitgourmet.com:
Steamer

I tried it at home with a commercial muffin mix. I was impressed. I was expecting something sorta mushy like a dumpling. Instead, the muffin was baked nicely and was done very quickly.

Now for questions: what can I use this technique for? Can you buy muffin/brownie mixes that don't need milk or eggs?

Sure. There are lots of add-water-only muffin mixes. They aren't in every store so you may have to make a couple stops. You can also add a bit of powdered milk to those that call for milk to make them add-water-only.

Brownies are a little harder to find. Try looking at the Fat-Free brownie mixes. Many of those don't call for eggs.

As Bannock points out go for the fat free brownie mixes. The Krusteaz one works well in a bakepacker. Also, their blueberry mix works well. It comes with a can of blueberries but they could be replaced with dried ones from Trader Joes. Haven't tried doing it in the muffin cups but with the bakepacker we also do bread.

Krusteaz mixes are good, and there are some cornbread mixes too. I bring along a little bag of powdered milk, and do use mixes that ask for eggs. Sometimes I have enough extra eggs to use them, other times I just add a touch more water and a bit of oil. Seems to work fine.

I've used Betty Crocker "add water only" muffin mixes and they worked great. I've found them at Target as well as Byerlys.

This thread reminded me of this, the Freezer bag cooking lady because she came up with the idea of how to make your own, lightweight, steam cooking method. The link also gives you some steam cooking recipies.

I, as of yet, have not used the steam cooking method....yet.

I'm a BakePacker guy too. I use powdered eggs and powdered milk in mixes that call for those -- but the only add water are just as good. Ditto on the Krusteze stuff. Cornbread is very good (goes well with the Bear Creek Chili). Bread, biscuits, muffins, pancakes, cakes (strusel and coffee cake variations too) -- used the calzone recipie last year too from the BakePacker - good reviews from the high school son.

all of the "add-water-only" mixes use powdered milk and eggs and other stabilizers in them to replicate the mouth feel and flavor of the reel things. just an FYI