To explain a little more, the combat map right now is really just a single dimension—units are X units to the defending or attacking side of the center of the battlefield. It doesn't comprehend the idea of them being also spread out in space along their line.
What I want to do is give it that extra dimension (literally) so that it can be more of a grid. That will enable a number of cool things, but I'm a little concerned about the complexity it will require for players who want to have any meaningful control over how their units use that space.
The simplest I can think of is, indeed, something along the lines of setting your unit to the Center, the Left Flank, or the Right Flank.
(Of course, it'll add a lot of complexity in the code, too, but that's my problem, not a problem I'd be handing to the players.)