They lost with space marine because of prior art as I understand it, the concept of power armored marine had been around since at least Heinlein.
For abandonment, then it depends on terriroty, but from a quick google, in the us the deal is:
The courts are of the view that rights gained under the Copyright Law may be abandoned. Abandonment of such rights, however, must be manifested by some overt act indicative of a purpose to surrender the rights and allow the public to copy. [Hampton v. Paramount Pictures Corp., 279 F.2d 100 (9th Cir. Cal. 1960)]
So it would require them to actively abandon it. Case in point, because Ford stops making a car, does not mean i can release said car because it is out of production. If GW chose to walk away from BG and say as much, then it might be different, but I am not sure it is a grey area. I think in fact it is basically not legal, but what is grey is whether they would bother to enforce their copyright. That said, knowing GW they will basically screw anyone for 50p.
If you think about casting, i think you are more akin to using Bit Torrent to download old TV shows, i.e you can probably get away with it, than to publishing Shakespeare because it is out of copyright.