Foxhack: Not really.
If you can score a used physical copy of the game, you can register the key on Steam and download it there. I got my key from a new copy I picked up at a local flea market for $4 or so YEARS before this whole mess even began.
Wishbone: If you find a physical copy of this non-Steamworks game, why wouldn't you just install it from the disc the normal way, rather than trying to get it to run (and download) under Steam?
That said, I have got a physical copy of Prey, and I think it's a fantastic game, so I for one am psyched for a possible sequel.
Well, the original version of Prey was available via a download service called Triton, which died shortly after the game was released.
The game was set up on Steam for people who wanted to use their Triton or retail key there, and I thought the retail version required this Triton thing (I was mistaken.)
And you know, for some of us, Steam isn't a problem. Jeez. :P