My understanding is that a lot of these engagements go an hour (or even hours) with virtually no casualties -- I'm not sure what the odds should be to match reality, but I suspect they're fairly low relative to the odds of being hit most systems I've seen would model.
That's quite right. With any single gunshot, the odds of getting hit even if the shooter is a few yards from you are actually quite low. Thus, one dice roll per shot is really really boring. Which is why you don't usually see these sorts of rules in rpgs - plus, who wants their character to be knocked over by some random unaimed shot? It's like stepping on a mine, maybe realistic, but pretty anticlimactic.
With large-scale battles what I've done is to move to "long turns." Whatever the length of the regular combat turn, the long turn is some minutes. The to-hit rolls then become about one of the very many shots fired happening to hit. For example, even if there's only a 1 in 1,000 chance of a hit, if 100 rounds go downrange, there's a 9.5% chance
at least one of them will hit. I'd rather make one dice roll aiming for 91+/100 than 100 dice rolls aiming for 1,000/1,000.
It's similar in ancient and medieval melee battles. A large battle isn't simply a bunch of duels. It's about groups of guys lining up shield-to-shield, the whole group moving forward banging their spears on their shield, then maybe one guy jabs at another in the line, that guy stumbles a bit, his line falls back a few steps, they're urged forward again by their commander, and so on.
With the long turns you then slot in a few duel-type actions for the PCs to keep the players happy, and usually you'll be able to say that however the PCs go in their duels, this determines or at least influences the battle as a whole.
In my view returning suppressive fire should be more risky than hiding behind a rock, but way less risky than advancing.
That's what you have cover and perception rules for. And rules about ease of being spotted. The enemy is laying down suppressive fire - but hey, if the enemy sees a guy get up from the trenches, they'll turn that suppressive fire into direct aimed fire. But they have to see him. Now, if he gets up and charges, okay he's easy to see, bye bye Pte Foolhardy. But if he slowly eases himself over the sandbag, just his head, shoulders and rifle, well maybe they'll have to make a pretty difficult perception check to notice him. But
he'll have to make a similar check to spot the exact source of that suppressive fire. And so it goes.