Now that I have the new and improved replacement METAL flange w/nipple,
instead of an o-ring, a flat gasket is used for the seal against the
cylinder head. What, if anything, should I use on this gasket?

I highly recommend a high tack sealer at this compression joint.

I personally use Caterpillar fire ring high tack.

Cat uses this sealer at the top of the cylinder liner's on the fire ring
in any of their diesel engines.

It works well, and the Vanagon compression rings won't ever leak again.