I made a loader/keygen that should work on future versions of burp suite pro, unless they specifically target it/change the license scheme.
Download loader/keygen here: http://www36.zippyshare.com/v/WRy3bGWf/file.html
Works on 1.6.x up until the latest version 1.7.31 and maybe into the future ;-)
My approach is a bit different than larry_lau, and my jar is not obfuscated so you can easily verify that it is safe :-)
You still probably need to firewall off the program like Sakaroz said, if it really does do license callbacks. (I didnt look)