Add Vanillacryl to your list of products that coat foam. Its in the same price range as more typical products, but it seems to be marketed towards the special effects / haunted house crowd as opposed to the theatrical industry. (By the way, the haunted house industry and model railroad industry often produce some great products that are very useful for theatrical purposes). You can find more information about this product at http://www.fxsupply.com/vanillacryl/vanillacryl.html
A few notes: Its water based, weather resistant, you can mix texture into the product, and it may be thinned slightly. It doesn't achieve maximum hardness for several weeks, though it can be painted prior to that.
It would be interesting to do some side by side tests on the various coating products and various foams. Of course then the next step would be to have a wear test for them to see how they hold up after a month of use.