Testors dullcoat never seems to make a dead flat finish, but I usually am not after that. I am not aware of any enamel clear flat coats out there.
For flat clear coats i use the Vallejo matte varnish 70.520. which is acrylic. Use too much and it will frost, and it will not at all protect an enamel base coat from oil washes. So yeah, there is definitely a need for a good clear flat coat out there.
I've just used Alclad aqua gloss and love it. If only applying a few light coats it will not 'gloss' the surface up too much. There may be merit in using the Alclad clear flat coats, even those are laquer based.