Yes it's legal. Here's an excerpt from ALPA's Guide to Flight Time Limitations and Rest Requirements, 6-A Edition June 2004. I believe there is a FAA interpretation for this as well but I couldn't find it.
I reference this guide because the example they use is specifically a domestic trip.
My company choose to not use .513-.525 as well, I think because of the additional rest requirements and limitations that come with it. We too use .507 and .509 for domestic trips, and your question came up years ago when we started using .507 for these trips.
FAR 121.507 provides:
(a) No certificate holder conducting supplemental operations may schedule a pilot.
(1) For flight deck duty in an airplane that has a crew of three pilots for more than eight hours in any 24 consecutive hours; or
(2) To be aloft in an airplane that has a crew of three pilot(s) for more than 12 hours in any 24 consecutive hours.
(b) No pilot of an airplane that has a crew of three pilots may be on duty for more than 18 hours in any 24 consecutive hours.
Q-84. An MD-11 aircraft (two-pilot crew) is scheduled to operate New York-Los Angeles-New York for 10:30 of scheduled flight time. An additional pilot augments the crew. Can this flight operate?
A-84. Yes. The supplemental rules limit flight deck duty to eight hours so with three pilots and 10:30 of flight time, the air carrier can limit each pilot’s duty at the controls to less than eight hours. If the duty time was less than 18 hours, then the flight could operate.
Of course this is all true for now, until the new FAR's take affect in Dec. 2013.