DaveC wrote:
So I guess without some custom "manifold" or such, there's no way to fit a
dip tube, internally. All I have is threaded holes in the tank for 1/4 inch
input, and 3/8 inch output ...

I assume that your 3/8 threaded hole is NPT. In which case, use a
npt-compression adapter/union, 3/8 NPT to 1/4 (OD) compression. Such as
McMaster-Carr #50915K316:
http://www.mcmaster.com/
But also generally hardware-store available.

Drill out the adapter to 1/4" to allow the tube to pass through it. The
fitting has plenty enough "meat" to do this, but as-made it stops the
tube for ease of assembly.

Screw the adapter to the tank, pass the tube through it, tighten the
compression sleeve/nut, add a compression-fitting valve to the tube. If
you file a notch on the in-tank end of the tube, you can lower it all
the way to the bottom and still have an opening for the water.

If you first fit a 3/8 NPT tee to the tank, with the straight-through
vertical, you can take your tank output from the horizontal leg, while
fitting the dip tube adapter to the top. The dip tube will be co-axial
with part of the output path, but with enough room for air flow.

HTH,
Bob