Are the icon files as "full up" as the oobs?
Can new icons be added reasonably easily?
My thinking is this - across several threads there have been comments that some units like Brandenbergers, certain panzer variants, captured armour etc take up a slot but are rarely used and could be easily represented by manually building the unit in the scenario editor. A hopefully simple solution would be to create a set of "User" icons that could be grafted onto an existing unit to create an new unit. For example - to get a German Somua, load a French one and switch the icon to the German one from the User set. For the various panzer variants, tweak the values of the closest unit and switch the graphic.
Same for armoured trains of course
AI wouldn't be able to make these picks of course but that should be a minor concern for most battles.