You're overthinking this. First, you can simply take a meter reading through your polarizing filter at whatever orientation you like and base your exposure on that (I do all the time with my spot meter). Second, the amount of difference in overall exposure between the filter in "unpolarized" and "fully polarized" settings may be 1/3-stop or a bit more, not really that much to worry about. People (me included) shot a lot of slide film with TTL cameras and polarizing filters on the lens and did just fine... For negative film there's even more latitude; when in doubt, overexpose a bit.

Best,

Doremus