If you want to use the pressure tap from the back plate, or better yet, the pressure tap on the carb body (best) , then the fill vent is not needed. You just invert the tank and fill through the uni-flow line that goes to your pressure tap and the tank is full once it comes out the feed line to the carb.

A large syringe and a wooden cloths pin is all you need to fill a pressure type bladder. I put the bladder in a balloon in case it bursts. Use the clothspin to clamp the line, and release it once the engine starts.

Another set-up it to use a small balloon inside a tank. The balloon is wrapped and sealed around the outlet fuel line. The vent goes to your pressure tap. First, blow on the pressure line to collapse the balloon inside the tank, to get all the air out of the balloon, Fill the ballon via the outlet line from the tank. When the engine is running the pressure from the pressure tap will push on the ballon and force the fuel in it to the engine. I beleive they sell kits for this, as my pal Larry Renger tells me the control line guys use this set-up.