There is plenty of grain-free food out there; it's even a search option on chewy. The problem is the grain filler is often replaced by something else that's either not nutritious for cats or possibly even harmful (like potato and peas), and that still doesn't resolve the moisture content issue.

Cats "drink" water by eating wet food mostly. Feeding them dry food in any capacity will hurt their kidneys. They aren't used to drinking because of it and even if they do, it usually isn't enough. Most of the materials I was given for fostering / adoption says to try to switch them to wetfood and keep them on it because of the reasons above.

As to your question - there are freeze dried meats that can be given to them, but because of the kidneys thing, you aren't supposed to give them more than a few cubes a day. Iirc 15 or 20?