[69hugger]

I would think you could fairly easily make a system to pressurize the tubes. use a flat plate over the head flange & make or buy a collector flat plate. Drill a hole in the collector plate & install a pressure valve (maybe just a rubber tire valve). Hit it with 20 or so psi & see if it holds pressure. Might have to use gaskets or silicone to get a seal. You can always spray soapy water to find pinhole leaks.
A bit of a hassle, but as you said, good to check them before coating.

Good luck!

Bill