mIRC does not listen to numeric 328, which seems to be related to the displaying of an URL. I'm not sure if this is RFC-approved (I couldn't find it anyways), but other clients seem to correctly use this numeric (Weechat, Irssi, HexChat)

Code:
@debug
-> availo.esper.net PART #zarthus
-> availo.esper.net JOIN #zarthus
<- :Zarthus!Zarthus@zarth.us PART #zarthus
<- :Zarthus!Zarthus@zarth.us JOIN #zarthus
-> availo.esper.net MODE #zarthus
<- :availo.esper.net 332 Zarthus #zarthus :snip
<- :availo.esper.net 333 Zarthus #zarthus Zarthus!Zarthus@zarth.us 1408901904
<- :availo.esper.net 353 Zarthus = #zarthus :snip
<- :availo.esper.net 366 Zarthus #zarthus :End of /NAMES list.
<- :services.esper.net 328 Zarthus #Zarthus :http://some.url
<- :DerpServ!Herp@derp.services.esper.net MODE #zarthus +o Zarthus
<- :availo.esper.net 324 Zarthus #zarthus +CFgnt
<- :availo.esper.net 329 Zarthus #zarthus 1395694735

Showing up in client as
(19:33:27) * Attempting to rejoin channel #zarthus
(19:33:27) * Rejoined channel #zarthus
(19:33:27) * Topic is 'snip'
(19:33:27) * Set by Zarthus!Zarthus@zarth.us on Sun Aug 24 19:38:24 2014
(19:33:27) * DerpServ sets mode: +o Zarthus

/msg chanserv help set url (Atheme)
(19:27:18) -ChanServ- ***** ChanServ Help *****
(19:27:18) -ChanServ- Help for SET URL:
(19:27:19) -ChanServ-  
(19:27:19) -ChanServ- SET URL allows you to change or set the URL
(19:27:19) -ChanServ- associated with a channel. This is shown
(19:27:19) -ChanServ- to all users joining the channel.
(19:27:19) -ChanServ-  
(19:27:19) -ChanServ- Syntax: SET <#channel> URL [url]
(19:27:19) -ChanServ-  
(19:27:19) -ChanServ- Example:
(19:27:19) -ChanServ-     /msg ChanServ SET #chat URL http://slashdot.org
(19:27:19) -ChanServ- ***** End of Help *****