After a quick test, I think it wouldn't be hard to create a CHEAT (Controllable Hover Entities Area Targeting) selection box targeting feature.
The goal here would be to be able to use box selection while holding a hotkey to order selection to target specified enemy units.
This would look like this:
Select your army
Hold down targeting hotkey
Box select hover enemy units
Then your selected units would be assigned to target all units found in the box selection.