Then why not do those ?
Seriously though, the layer system as you laid out above is quite taxing and is not designed to maximize leg size. It's designed to build "The Power Look" which is really more of an upper body look, and the TBDL takes care of the rest.
Adding a sixth leg day and having only one day off a week could probably be done, but I dont know if doing so would be beneficial, or you might have to make it a 8-10 day cycle with a few rest days thrown in.
Obviously you can do whatever you want, I'm just giving you my advice from my experience on the system. Adding a full on Squat layer day would be adding a hugely taxing event onto an already fully loaded schedule.
If you had done HPMass, do you then have access to the Prowler? Maybe adding in 1-2 heavy prowler push/pull days after the SGHP's would help. Thats brings the leg stimulus up to 3x a week and wont tap into recovery quite as much since its eccentricless. This is actually what I do for my conditioning currently, so perhaps that is the reason for my leg size increase/maintenance.