[Saulot]

"gain a charge bonus to damage based on the attacking mounted unit's AP "

I could see that. Where both the pikemen and the heavy cavalry get killed when the cavalry hits with the charge.

In this case, the 'vs cavalry' bonus would be tied into an enemy unit having a charge attack and not a mounted identifier.

However, adding a new unit ability would probably best be done by Illwinter and not by modding. Anyway, it's an intriguing idea that should be carefully examined in a myriad ways by the community (Possibly better outside the modding section of the forum).