I think this would be a boss idea, and it'd give Khuzaits an interesting playstyle. At the same time, I don't see any viable counterplay to such a mechanic. It does seem like it'd be an unbalanceable "click and win" trick.
I was thinking you could copy the javelin mechanic on horse back. If the enemy unit has his shield up, it doesn't work, but if its down then he get his neck lassoed. And then you could ride off, but you could maybe add another requirement for the rider as he would need to tie his whip to the horse's saddle in order to drag the unit. If he doesn't then he'd be pulled off the horse. So there would be a little risk reward.
Also, dragging a unit would not kill them, It would require like riding from one flag to another distance. And the horse would be slowed down by this.
So basically, it would be like a javelin but you have unlimited amount. It would tricky to land your hit but if you did you could successfully pull out a spear unit from a safe distance.
This would create more fighting mechanics for the horse factions.
(Also I think the horse archers have been way too nerfed down)