Dave,
I don't see this as "gaming it" - and I'm pretty intolerant of those sorts of shenanigans in WFB.
Brigade-of-columns seems like the reasonable way to go for all-HTH brigades for the reasons you gave. Why would it be more "right" to deploy them all in line? The fall back rules will only risk jacking up the regiments behind the first.
Now if the fall back rules allowed the front rank to filter through the supporting ranks, with a leadership role to avoid confusion in the ranks standing fast, that would mimic ancient army tactics.
On that note, how does that work in Warmaster Ancients? Because Roman Republic armies were known to deploy in lines according to class and seniority (hastati, principe, triarii) and when the first rank was used up, the next rank pushed on through into the fight. They weren't displaced.
Mixed order (Napoleonic ordre mixte) seems like it would be good for shooting/HTH brigades. i.e. Archer regiment deployed in line of three companies with spears in column on either end. Have the archers hang back when the spears charge in or let the archers come in last positioned to support but not take direct attacks. Or just hang back and shoot at other unengaged units.
Apololgies for being Mr. Smartypants with historical references and the conjecture when I've never played a game of WM.

How does it really work on the table top?