You can do 2 things.
1.) Use an HTML "On Open" statement to launch a script that will check version. If the client is not installed, then it will redirect them to a page where they can download it. This is pretty much like the default NFuse page.
2.) Use the embedded Java client, then users won't need to install the client. There session will be embedded in the browser window. On the root page, you can add a link that can install the client at the user's option.