I think there was a mod for the Second Age.... try checking over at

http:\\www.twcenter.net

The mod may have some mention over there. It would make for an interesting mod though. The various races were not necessarily allied then. The total number of viable factions is smallish...but you can have the three main houses of men, plus various family factions of Elves.

As far as the dismounting of cavalry goes, it's not possible at all I am afraid.