it would be nice to make it harder to violate the license agreement
The answer out of left field would be: Build a better relationship with your client that is based on trust and honesty.
If you want to go the obfuscation route, then
ProGuard obfuscates web apps as well as any other kind of app. You may need to tell it about lots of class names that it should kepp which are not obvious from the code, though.
[ December 15, 2006: Message edited by: Ulf Dittmer ]