Are you using the ![CDATA[ejbql statement]] syntax in your <ejb-ql> tags?
No way to know exactly why you're having this problem without seeing your deploy-descriptors but sometimes it helps to just know that these do work.
I've used LIKE and all the other jbossQL specific operators as well as the <> on most versions of Jboss with mysql no problems - so you're doing something wrong.
If you're still having this problem post your ejb.jar and jbosscmp-jdbc.xml and maybe someone'll see the problem.