Well as my post said, I can't find any hardcoat that specifies it is "flat" and I'm looking for a product that is flat but thanks for your completely non useful smart ass answer. Since there was no actual useful information in your post, I can only assume you don't know of a product that will help me.
Edit: answered my own question. Krylon makes a Matte Finish product number 1311. It doesn't say if it's acrylic or urethane but I'm going to test it before using it. I'm sure a search of thier site would tell me exactly what it is but I want to make sure it's flat enough for my needs.
I apologize!! I was in a pi$$y mood and it was meant to be a smart A$$ answer. Now as far as a product. The closest you will find to a true flat in a clear coat, is what you have most likely already found, a matte finish. Flat paints are designed to be non reflective when cured, any attempt to sand or otherwise add protective finishes, defeat the purpose of the original design of the paint. Most flat paints do not look smooth for the reason that this introduces multiple minuscule facets that reflect or refract light in many directions ( that's in lay mans terms ). Thus the pigment and additives cause this effect and the pigments being non reflective achieve the flat look. The same with Satins, mattes, and semi gloss. They have differing degrees of additives to achieve the effect they produce. That's why when you open a can of clear satin it looks milky.
Again my apologies for my smart ass attitude. I wish you well on your journey of the Flat finish.
MK
edited to add: If you are using spray equipment, you might consider adding Transtint Black to your clear matte finish, it may help keep the depth of the clear down to a minimum and enhance the matte effect. Just a suggestion.