I'm with AC - I wouldn't dream of advising anyone to nonrev on our flights ANC-YVR from May thru Sept as I know they are almost always running at 100% LF. Someone at DL would know more but I'd imagine their (and the other limited ANC service carriers) also run close to 100% LF from ANC most of this season.

I'd imagine your best bet is to nonrev on AS or get one of the APEX or www cheapies on AS down into SEA. At that time of the year they run something like 18-20 flights per day ANC-SEA.
I wouldn't personally try to nonrev on any other carrier at that time of year from ANC down to the lower 48 due to the amount of people travelling and the vey limited frequencies the other carriers run. Once you're in SEA it's much easier to nonrev.