I'm still looking for a basic "white butter cream" filling that I
can play with for color/flavor, if you happen to know of one, other
than ganache.

A white chocolate ganache is a good neutral base, if that's what
you're looking for. Note that with white chocolate the cream should
be slightly cooler than with dark chocolate - definitely keep it
below bubbling, even slightly.

I'm interested in how you would make a white chocolate ganache. I have tried
several different approaches, but always find the texture not what I would
wish.