When specifying a hostname you'd need to use a specific switch. Switches can differ in location in the syntax, so there is no set syntax, no.
On DALnet for example, if you searched for the hostname 'arnie.eats.cookies.com' in all channels you were in, you would use /who +Mh *arnie.eats.cookies.com*. However, on Undernet, there isn't a switch for just channels you're in, and to search specifically for a hostname I believe it's /who *hostname* h.
Regards,