You can use a bag for both. The secret is that the bag MUST NOT touch the pot's sides. One can though use Food Vac bags for this purpose which are rated for submersion in simmering/boiling water.

Now though with steam baking all you need is water below to well, steam the items
http://www.trailcooking.com/thefauxbaker
That has all the pages of stuff we have done with steam baking. I prefer in most cases using silicone these days. It is easier to deal with.

But on the submersion of baked goods - while one could do it and it would cook, steaming gives a light product and works easy for great results

Also, Packitgourmet is selling new submersible bags now. I haven't tried them though.