That would be super.

Hugo says to look at http://www.mock.com/receiver/server/, under the "Console" heading, which details the pins on the serial header.

Is that the SSDP upnp UDP broadcast thingy?

Yeah, but it's not real UPnP/SSDP, so I've stopped calling it that.

Found music server

...means that it's booted the second kernel, and is now grabbing the player.

Searching for music... (inside a single-pixel frame)

...means that it's loaded the player software and is now looking for the HTTP server.

If I start the player against the Windows Audio Receiver...

If you turn it off with the button, it's only soft-off. It's not actually exited the player, so it doesn't need to make any more service locator requests, and will continue using the old NFS server.

Surely it isn't relying on some quirk in Linux's NFS server.

No.

Can you get debug output from your NFS daemon (or a network trace). It'd be interesting to see what files it's looking for (and whether it finds them) once the second kernel starts up.