Usually I just do aircraft, but having done hypothetical things with that subject, I suppose for armor I'd probably do something like puting wood planks on the sides of something like a Challenger or Merkava sort of like some of those pictures you see of Shermans with wood planks on their sides.
Maybe a WWI British tank with ERA bricks on it.
For a real anachronism, how about a horse drawn Abrams? How many heavy horses for that do you figure? :-)