With ruben01 we made an applet verifier to test which version of java someone is running and tell a custom message based on that.
The idea was to put our plugin2 applets on gamejolt by using the jnlp_href parameter, but show a nice message when user has no plugin2 enabled.
For example, the applet tag was configured:
<applet code="com.gemserk.games.appletverifier.AppletVerifier"
archive="appletverifier-0.0.1-SNAPSHOT.jar"
width="800" height="600">
<param name="jnlp_href" value="http://www.gemserk.com/..../somegame.jnlp">
</applet>
Then, if you have plugin2 enabled the jnlp is automatically downloaded, else the AppletVerifier applet is loaded, checks java version and shows a nice message.
I know this is a different situation, but I believe it could be useful so I like to share the code.