Is it reasonable to assume Jar Launcher is set up to run jar files from Finder on OS X 10.5+?
The Apple supplied java implementation comes installed with Mac OS 10.5 and 10.6 and .jar files are associated with the jar launcher application. As long as the java application starts right into a swing interface it should open without any problems.

On Mac OS 10.7 Lion however the java runtime is not installed by default and users trying to launch .jar files will be greeted with a dialog asking them to install java to continue (which requires admin privileges).

Officially the apple supplied java runtime is depreciated so you can't count on it working in any future version of mac os x