So I looked into this a bit more.  I wish I hadn't.

The active PnP resource information is stored in RAM onboard the
CS423[678] chip.  This memory is initialized either from a serial
E2PROM wired to the chip, or by the host (i.e. the BIOS).  I'm not
sure which is actually initializing it in these machines, but I dumped
the contents of the RAM, and there are no PnP logical devices defined
in the PnP resource section.  Given this, it's not terribly surprising
that it doesn't show up as a PnP device.

It seems that the only way to properly configure this device is to use
the PnP BIOS to locate it as a `system device'.

Yuck.