Thanks for correcting my posting Chris - one loses track of who said what & where!
I'm wondering if the B707 airframe, specifically the vertical surfaces, may have been merely adequate for the regs? Has anyone got a copy of Davies to hand?
Short answer: Davies doesn't mention structural capability so far as I can see. Boeing paper on use of rudder as published by IFALPA says that all Boeing aircraft meet the regs with a bit to spare, but don't say if that applies to just current range or to all designs past and present (KC135 excluded!)
Davies has quite a bit to say about dutch roll and recovery therefrom, also some interesting remarks on rudder bar travel cfd rudder angle. (p263 in my 1976 edition)