This is a little more work, but you can bake potatoes wrapped in foil in the coals, then mashed them up skin and all.  If you have any onions or peppers along throw those in, then just make into cakes and fry them up.  It works OK without an egg, but that would certainly hold them together better.  Suppose the instants would work fine, but you get a nice "fire-roasted" flavour with the real ones.