I think the hard part is the MOA at 500 yards in a .223
I mean most any quality .223 will shoot MOA at 100 yards...that's not the hard part...it's when you introduced that 500 yards thingy that things got tough. Even with match ammo and a sled or rail gun I wouldn't bet on MOA at 500 yards.
So, to get rid of some of the variables, to help you achieve your goal, a heavy style barrel might help...but then it blows your weight requirement...that's why I recommened a tension barrel....less weight but stiff barrel capable of rapid cooling.
"So is there any kind of rule-of-thumb, that says a rifle less than 12lbs (or whatever) won't consistently shoot worth a crap?"
No, I have a .223 Savage 111 Chieftain (9#'s w/scope and bi-pod) that will do MOA, but not at 500 yards even with match ammo.....300 yards...yes. I'd say consistent MOA at 300 yards isn't crap even if the rifle only cost me $250 new about 10 years and 10,000 rounds ago.