Well, the oats I already linked to (as an example) have been “milled down to an ultrafine powder ready to be mixed with your preferred amount of water or milk.” And I personally (and I doubt I’m the only one) don’t really mind if my meal would be smooth or lumpy. I’d happily eat something with the consistency of oatmeal/porridge nearly all the time.
You are correct, I already said this can’t compete on convenience. It can definitely compete on customisability and optimizing the formula for your personal nutrition needs. A great example is if I want to achieve my “complete nutrition” but in 1600 kcal worth of food - it’s very inconvenient to do that buy purchasing Huel as it is.
Edit: Here is another source of fine oats - http://www.bulknutrients.com.au/products/fine-powdered-oats.html. Bulk Nutrients looks like a good source of whole-food and isolate powders.
Edit2: And another - http://www.bulkpowders.co.uk/ultra-fine-scottish-oats.html.