I'd like to give access to some software to users via the X11 protocol. A browser plugin seems the simplest. Are there any alternatives to the above?
Xming is pretty feature-rich and exists as a portable version, although not a plugin. You might succeed in doing some preconfiguration for the portable to hand out (or make available as a download) to your users.
Edit: as you referenced Wiredx, there also is Wierdx which is an open-source java project forking off some old codebase of Wiredx.