The reason for 'no-waitlist' for earlier flight may have nothing to do with 1 coupon rule, but more to do with 'International Travel' as YYZ may be considered International for stand-by purpose.

In purely domestic iteneraries, the GA or CS agents can do it if there is an 'irrops' and they can create an additional coupon with a 'sup' giving an 'over-ride'. I have done it a couple of times (on domestic).