This would be very cool to have indeed.
I think the best way to implement it would be to have unit-type filters hotkeys. This way, you could have two complex armies, yet have their respective archers, spearmen, springalds, etc accessible. The most obvious way of implementing these would be to have preset filters in the hotkey options, but it might also be useful to rearrange which units that are on which filters as the game goes on. It’d be like sub-control groups in a sense that would foster better control of split armies. This would be an anti-deathball mechanic as well, which is nice.
I believe the main irreplacable function for the standard hotkeys is army position / intent ( raid, main army, defence squad etc), as unit types is something that’s generally a constant in comparision.