Long story short, the player spammed as many untis as he could and managed to place them in KHazar just before the Mongols arrived.
I did this a while back, to annoy the Byzantines, although I certainly wasn't the first because I got the idea from someone else on the Org I think. I dumped loads of peasants in by sea as the English in 1229. Then I went back to attacking Spain and watched the fun.

It does work. Stuff Khazar full of cheap units and you get a superhorde. The unit mix is the same as usual though, maybe it would have been better if you stock Khazar with good troops but that sort of defeats the purpose.