Haha, judging from the video the Greyhound is an excellent zombie-mower. Puma too probably. And who knows, maybe those hedgerow-cutters on the Sherman will come in handy one day:)
Seems like a cool mod, but really needs flags/goals as stated in your idea. Now it just looks like you'll be mostly walking backwards. Also, more /better zombie sounds, but I guess you're working on that.
But the promise of the epic arsenal of FH2 against mindless opponents is kind of mouth-watering. Far fetched, I know, but maybe you can add classes to the zombies also. A deranged pilot for instance. The zombie pilot would be barely able to fly anymore (but he remembers somehow...) and will mindlessly try and dive his plane into your vehicle if you don't shoot it down first. The Stuka and his siren could be quite frightening that way.
Or maybe a randomly placed zombie tank crew stuck in a tank that's broken down but can still fire. Either directly on the escape route or somewhere in the distance where you don't expect it.
Anyway, the idea is that the zombie opponents basically try to continue what they were doing before they became zombies. An engineer could be the Boomer type. With satchel in hand he could be a quick end for you and your crew but if you manage to shoot him first he'll blow up and take a pile of his zombie homies with him. (He should be rare, but the effect could be something like the scene at the Helm's Deep siege in Two Towers where the Urukhai with the torch tries to get to the wall to light up the explosive charge and the panic it causes with the defenders).
And so on, and so on. Good luck on the mod. Will try it out soon.