I was a painting contractor for 13 years and I'm telling you-yes they do make it. I'll hunt around for a brand for you- I'll call some of my buddies who still paint. Before anyone sounds off, you need to know a few things.
1)If there is a super flat paint, then there is a super flat clear coat. ALL ACRYLIC CLEAR COAT IS, IS ACRYLIC PAINT WITH NO PIGMENT IN IT! SO YES IT WILL BE JUST AS FLAT.
2) If you're using something with a sheen-QUIT USING IT.
3) Tony, I hate to disagee but think we can do it without being diagreeable-just because you haven't seen it doesn't mean it doesn't exist. I have one- Except they screwed up the stripped lower and gave it a satin- which still has a little sheen (They did such a nice job I didn't want to complain)
4)There is nothing magical in flat paint that can not be replicated in a clear. It doesn't matter if it's latex,oil,alkyd,acrylic etc.. unless you're talking about varnish it's a matter of having pigment or not having pigment- Any paint can be reduced to its' base and still provide protection.
For whatever it's worth,
Brendan