Alternative to applet - launch from browser

I recently found an interesting Java applet that works great for electronic signature, but due to recent NPAPI stuff with Chrome browser, I figured it would be nice to port this applet to a Java application. HTML5 / javascript / etc is not a feature.

A special requirement is that the application must be launched from the browser, even though it is not an applet.

The best approach I've found is as follows:

• Create an installer that will create a java executable on your local PC.
• Create a javascript function to detect if the app is installed on the computer (just like iTunes does / does).
• To launch the application, create a custom URI scheme handler that will launch the application when it is detected by sending the required parameters to the application (as an applet does).

Everything sounds pretty smooth, but it has certain disadvantages:

• It is designed to work on different computers. This means that the URI scheme handler and application MUST be installed / configured on every single computer I want to use the application on.
• Porting the applet code to a Java application. This is an old application that I didn't write, and it got one class with over four thousand lines.

With this in mind, I couldn't think of a better option. Is this viable? Any recommendations?

Thanks in advance.