John I have several units based up as Irregulars, for which I mount just 3 figures on a standard 40mm x 30mm base (15mm Figures), with extra scenic treatment as befits their mode of use. Certainly beats using Regulars with staggered bases from an aesthetic point of view.
I agree that the Irregular units rules seem a bit odd conceptually. On the frontage issue (which one would think is wider) I rationalise it by assuming that a company or two is at the rear to provide formed supports as was common practice in many armies. This means fewer men are in the firing line hence the same net frontage as the Regular unit.
The part that is harder to visualise, or to think of real life examples of, is Irregulars in square as you say. Also you would expect a little more in the way of manoeuvrability than Regular formations.
When playing scenarios I often allow eligible units to break down from Regular to Irregular taking 1 turn as a formation change and to reform taking 2 turns. I also ban them from forming square while in Irregular formation although they can form into square as part of their 2 turn transformation back to Regulars.
Lasalle has a simple core of rules without too many exceptions. To my mind that is a strength, and one of many reasons why I like them. For me it also means that when I want to I can add the bits of chrome I need for a particular scenario by making up some house rule for that game. I have read with interest your blog entry on Hougoumont. I hope you explain all your adaptations in detail as it progresses.