In patients with skin squamous cell carcinoma invading the skull, what dose do you limit the brain when treating definitively with radiation?

Brain (excluding optic structures and brain stem) can handle all sorts of doses to limited volumes.

I would just try to avoid dumping hotspots < 105-110% of your Rx dose into the brain and try to minimize the Rx dose, which will be dependent on anatomy and the extent of abutment/overlap of PTV with ...

I use 1.2 Gy/fx BID to 72 Gy to treat the gross disease invasion/perineural gross disease. That translates to a dose to the adjacent brain of EQD2 of 67.2 Gy for alpha/beta = 10 and 60.48 Gy for alpha/beta = 3.