Any ideas if this client will work with other servers?

It doesn't. The Imerge client (NP200) and the Onkyo equivalent (NC500) will support servers running the latest incarnation of XiVA software.

Onkyo have a Windows utility to stream from a standard winows box too.

Both can access Intern radio stations directly via a router.

Differences between NP200 and NC500:
NP200 has optical digital out (with SCMS protection, unfortunately); RDS for FM radio.

NC500 has TV output for UI (not capable of streaming video, though).

Indeed, protocol it runs is Onkyo's proprietry "Nettune" over standard TCP/IP

Cas.