Even if you are getting fluid out of the stem, if you seat the gauge on the valve stem, you will still get an accurate pressure reading. You can always bleed off the fluid till air/liquid mix starts coming out and then reinsert the valve core. There is practically no way to completely fill a tire unless you put some kind of upturned tubing into the stem to allow any air above the core to be forced out AND who in their right mind would do something like that. Once you bleed the air out so you are getting water from the stem, one could pressure the tire up to 20 PSI with additional liquid and perhaps fill 1/3 of the air cavity with liquid before the pressure build up would force you to stop filling.
I fill my own with pure water from my hose (not to much danger of freezing here). I listen for the sound of the water to change indicating that the stem is under water, remove the hose, bleed off the water till it starts to sprit out mostly air and then install the valve. Then adjust the pressure with additional air as required. One could do the same way by pumping in a proper amount of pure anti-freeze first, then hooking up a hose to finish filling with water.
Perhaps a bit off topic: But, my calculation of cost is about equal for using anti-freeze at $10 per gallon and methanol/water mix and might be a bit cheaper for using rim-guard if you are looking at -40 protection. For this temp you need 50/50 mix so a tire that holds 100 gallons would be $500 in antifreeze. Somewhere I heard that rim guard could be had for $3.50 per gallon so that would make it much cheaper than antifreeze solution for max low temp. At higher freezing temps one would find a break even point in cost between the 3.
