I'm experimenting with creating puff cereal at home. I was wondering if creating a gel, dehydrating it and then puffing it in an oven with high pressure is the only way. I actually had an successfull experiment with making a puff cereal mix mainly made of cooked starch, dehydrating it and then putting the dehydrated balls into the microwave. The remaining moisture made the balls puff into puff cereal and it actually worked quite well. This is a technique used for shrimp crackers as well. Now here is the catch. It works well, but only with a few balls at a time (one ball works best). This is due to the uneven distribution of radiation which causes some balls to puff quickly and others to take forever, while the other ones burn. So I'm thinking about other ways to incorporate air into the gel without using something like egg whites which would change the texture of the final product. The goal is to avoid the uneven cooking of a microwave oven while also avoiding the need for a professional puffing gun or extruder.
You can create a "puffed" 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.
While I can't talk directly to the idea of adding air to a starch, I can mention some experiments I have done. Perhaps you could try whipping your starch mix before dehydrating to capture and trap some air.
Note that many puffed cereals are puffed due to the expanding action of steam due to small amounts of water (puffed rice, prawn crackers, popcorn).
I have experimented with using a small popcorn maker (hot-air type) to puff up things apart from popcorn. The popcorn maker is effectively doing what you want, but optimised for popcorn.
I have tried prawn crackers, and they are normally too large, but puff up quite well when broken into smaller pieces. Sometimes, the pieces are so light that they blow out the top of the popcorn maker; in this case, I place a metal sieve over the top to prevent this (but allow the air to escape).
I would try this with your cooked, dehydrated balls, aiming for a similar size/weight to popcorn kernels.
NOTE: the hot air can be enough to set things alight. I would try things outside first (lest there be smoke or fire), and don't let anything dusty into the popcorn maker (think dust explosion!). Aim away from eyes and face. I have set off the smoke detector numerous times.
This is honestly a shot in the dark, but could you try an air fryer? Google says they work for prawn chips.
The air fryer is meant to mimic the intense heat of a deep fryer without the fat. So if a friend has one, it might be worth a shot. It's obviously another piece of equipment, but nothing commercial grade.
That said, have you attempted baking with any leaveners like baking powder or baking soda? Those would give you some puffiness through baking, though I can't guess on the amount you'd need for your recipe.