My tanks have the fuel hose go all the way to the rear, but they have a couple of holes in the tube at 1/3 and 2/3 points to prevent the balloon from covering the pickups.

If you really want to do it right, get a short length of dowel that fits the balloon nozzle tightly, drill a hole in the center, epoxy copper tube in the hole with enough sticking out of each end to connect fuel hoses. Coat thoroughly with epoxy to fuelproof. When you bind the balloon on, there should be no chance of air leaks.

If you have a lathe, you could turn the whole assembly in one piece including nice fuel nipples on both ends. Probably could 3D print such a thing too. 

Might try the bladder nipples from MBS Model Supply.

I think John R. is talking about slack balloon tanks, where the balloon is never stretched.  It just makes a "fuel cell" that delivers fuel at atmospheric pressure, but without foam or bubbles or whatnot.

Bladders are a different thing.  Kind of a Hilborn injector vs. a carburetor thing.