I can only suggest looking in this list XMPP | XMPP Clients Though some of them might be outdated or discontinued (like Jappix, JWchat). I’ve heard people using Converse.js to integrate chat on their websites. But don’t know how many will work on IIS. Web client is no a popular thing in xmpp it seems.
I’m in the same situation as you Dennis. I used an old version of Jappix (0.0.0.6), found in Index of /Utiles/Jabber/Openfire-3.8.x/Plugins/2013 since the official jappix’s github page does not have the plugin for openfire (it redirects to Dele Olajide project as I explain below).
There is, also, another github project from Dele Olajide, but it is only zip and source files, there isn’t any .jar/.war file that you can use to import to openfire in order to install it. Dele himself, who is the creator of Jappix’s openfire plugin and several others openfire plugins, told me to use sparkweb instead of jappix in an email reply. However it is the same situation, outdated.
Although Jappix 0.0.0.6 is outdated and not supported anymore, it resolve my demands, and may be the solution for you too.
Candy is up to date, however it is used only as a massive chat room, which is very different from Jappix and its one-to-one chat.
I suggest you to migrate from openfire to other XMPP server, like ejabberd, since it supports other XMPP browser-based client chat like JSXC and I guess even Jappix. Jappix suggests to use XMPP server called Metronome, once it was specially developed to support Jappix.
If there is no specific integration or bugs, any xmpp server should work. Openfire also does BOSH like ejabberd does. Unless as i’ve said there are some bugs or not complete implementation, which may force to use some particular server.