Thanks all, for (respectively) starting or answering this thread. I'm also thinking of this kind of jawfish (from KP aquatics no less).
I've heard 3" inches is all that's necessary before (in contexts where other people say 4-6"), since the fish will arrange the sandbed as needed for their dens. But this is the most interesting:
Are you saying that, basically, I could even have 1" or so sand, and if I provided enough rock rubble in a pile somewhere that would be good enough for it?
If so, I assume it's pretty small rubble. Is such rubble commercially available? I presume, say, that Marco Rock's dry rock rubble is too big (at least as shipped, I could always break it down even further........):
http://www.marcorocks.com/dry-rock/rubble-rock-any-amount-order-by-the-pound/