Is there a way to incorporate air into a gel made from starch and will the pockets of air stay when dehydrated?
You can create a &quot;puffed&quot; texture with methylcellulose.  Chef Jose Andres made a popular dish, based on this concept, at one of his restaurants, using beetroot.  Methylcellulose is whipped into the base, and it foams like whipped egg white.  Then you can pipe the mixture onto a dehydrator tray and dehydrate.  There is no need to bake or cook after that.  I am not sure how well it will work as a cereal. It might not hold up well in the presence of milk.
It is also possible to puff many grains simply by overcooking (to gelatanize the starch), dehydrating, and deep frying.