I have visited at least one aquarium that had a tank of live horseshoe crabs. I had to be pulled away because they are so damn fascinating to watch. Anyway, it seems, based on that, that farming them might be possible. Now you've got me wondering, though, how the aquarium maintained their display stock.
And even if they could be farmed, or, for that matter, even if we could synthesize their blood, I'd still want their damn habitat protected.