Number of years ago I shot a 1992 Dodge dually diesel a flat black. I really liked the look but didn't like the maintenance of flat finishes. I am now doing the same to a 78 GMC square body. What are the thoughts of using a flat clear coat to make (hopefully) maintenance a bit less tedious?

The ONLY matte clear I would consider as normal maintenance/wear/tear product would be Tamco's matte clear. It is essentially their 2104 high build clear with a very unique flattening compound blended througout it. You would know the difference when you even open the can. You can see the flattening agent blended right in from the top of the can to the bottom with NO settling out. They use a silica as that agent which is similar to a product we use in high end museum restoration with furniture. Here is a link.....
https://tamcopaint.com/products/matte-clearcoat
Okay, so what is it like in day to day use. We are coming up on almost 2 years since I completed my truck conversion. I drive it about 8000 miles or so 2 to 3 days a week all year round. It has shown no signs of chipping, delaming, or just about anything. I do use Dr. Beasly's Matte sealer on it about once every 4 months or so with no washing at all in between. I just wipe it down with a matte detailer every couple of weeks and that's about it.
I'll put up the most current pic. I have of it here and also the pic. of just after we shot.... I'm telling there are just no signs of wear/tear/color shifting, etc....

there is only one matte clear i would use on a complete, PPG Global D8115

You can denib between coats, very even finish easily achieved by most people,

Dr. Beasly's Matte sealer is also what id recommend