I know how to use a GCV, I know how to use a local variable.

Can I read a value from the driver configuration? Say Authentication
information (user or password or host?) via Policy?

I suppose I could read out the raw XML of the DirXML-ShimConfigInfo to
get it, but is there any other way?

Passwords I imagine would be a special case and should be a
named-password or else not available.