I don't do brisket flats alone BUT I do chuck roast (when its cheaper than brisket) and our cuts are often big and long just like a brisket flat without the top layer of fat.
I firmly believe that the magic comes in wrapping the chuck (flat in your case) in foil while splashing some liquid in with it (I use left over wine or some beer) at a meat IT of about 185-190F.
This allows the brisket to get good flavor, color, and some bark while coming out tender and not dry! Brisket flats and chucks will wanna dry out on you being so flat and not full of fat.
Again I think that is the magic needed to get what you are wanting without it turning into pot roast or going some other unintended direction.
I smoke in my MES at 275F so I think you may be able to go any temp you want as long as you understand the fundamental issue of that cut of meat just wanting to dry out on you, hence the splash and wrap at 185-190F.
I hope this info helps and I look forward to seeing what you make :)
Appreciaate the additional info tallbm ..Picked up an 8.3 lb flat today that had a nice amount of marbling in it..The plan is to smoke it on Friday..I'll post up my results..stay tuned.