Actually, I think it would be quite possible at Regent or even Monarch, but you would have to build a tech lead and then use a tank/arty stomp. Supporting a 200 or even 300 unit military in OCC is not that hard in the modern age - the problem is the time it takes to build them - so you'd want to continuously build defensive, horse, and bombard units as much as possible throughout the game to have a large force of cavs, infantry, and arty when you get replaceable parts. You could probably start the stomp then, and switch to all out tank production when you get motorized trans. The key is to be able to produce 20 shields per turn at size 12, for 1 turn spearmen and catapults. You will likely be very resource-poor, which means you can still build spearmen well into the industrial age. The introduction of the trebuchet throws a huge monkey wrench, though, as you need 2 turns per bombard unit once you get Engineering, but still do well enough. I think I'm going to try to do this, and see what I can come up with. I currently have a OCC game where taking over my continent would be a cinch, so I'm pretty confident it can be done.