I bought some mantic bits from hoard o' bitz(s?) on ebay. They had a smattering of the warpath range, and more of the kings of war stuff. I'm not sure if the corporation arms would go on a guardsman, but other truescale 28mm stuff probably.
I think that there are some useful bits throughout the mantic ranges that people can make use of. It just isn't necessarily directly compatible with
40k part for part. Corporation marines, for instance, make great stand-ins for guardsmen... but I don't think they can swap parts without looking silly. That probably hurts the demand somewhat.
Fantasy though, may be some more opportunities there, although its just cheaper to buy your entire army from mantic unless you want something they don't carry