Base the spell on an Auto-castable one with no actual effect and have a dummy caster actually cast the effect of the spell on the (Target unit of ability being cast) when an unit casts the auto-castable one. Unfortunately I tried something on an auto-castable spell with version 1.15 and the trige was activating only if the player was activating it himself (eg: Slow an unit by clicking on the icon and then on a target). Hopefully this problem was solved, but I don't really know it.
This concept comes from Natures Call: Circle Defense. What darky did was create dummy spells, and when the target was selected they would add the ability to the caster, cast it, and remove the ability. Credit Darky for that one
Have you even played NCCD before? You would see with the Zap, Boulder, and various towers that you would see a new icon added and be cast, then removed. This works pretty well other than the fact channeling spells would be a bit weird
Nope... I played only "RotD" which I must say it's medium. But spells are indeed pretty good though they are not veyr useful. I hate spells which are good but not useful.