There is no mod support because GW hates mods. That said, in the official wiki at the end of each of the 4 base factions' rosters there's a note saying more units will be added as free-LC. And in the free-LC plan thingy, the first to be released will be a unit (to VC, based on the background of the picture? I hope it's Blood Knights.).
I think TW:WH is a different case. GW didn't like things that potentially ate into model sales, or into getting license fees from part II of a series. WH is dead, they're fine with CA having plenty of creative freedom, so it's up to CA on whether they're fine with modding in general. Modding WH related stuff doesn't take a single cent out of GW's pocket (WH is dead TT/sale/model-wise), and good mods might even sell a few more TW copies for the brand. It's CA's call, not GW's on this one. They stand to lose DLC sales if a mod already covers that, or does it better.
For pump wagons, the basic mechanics are already there for cool pump wagons. Winds of Magic "pulsing" UI elements, with moving safety thresholds being depleted for speed boosts, and ever increasing chances of damage. Walk/march/charge lowering or recharging available "winds" and safety margins (so go too fast for too long and you'll crash/cop damage, and they're not too sturdy to begin with). PoV for racing/fun. Cooldown "skills" for jumps, spores, and maybe handbrake turns, all lowering the safety threshhold for a while, but letting you be "Snotling Awesome" at key times. Unit upgrades for the damage upgrades.
It just depends on whether we can get access to them. Hacking in a restatted chariot with a different model will probably be pretty easy. But making it fun and unique isn't out of the question, it just depends on how hard it will be. Can't wait to peruse those game files.