In a scripted bundle, the activator can be just a singleton class, and GroovySystem can be used to create a fancy singleton pattern in Groovy, like it is described at Groovy's homepage.
The problem I faced when I tried to initialize the scripting engine using javax.sript API, was that the engine implementation still thinks that groovy.lang.MetaClass is a class, but it is an interface in the latest version of Groovy.
So the stack trace of this exception was:
Exception in thread "main"
The quickest fix I could do, is to download the scripting engine sources from the website, and comment out the line in GroovyScriptEngine's static initializer. After the recompilation I didn't get the stack trace any more, but I think I have to investigate a bit more, what could be affected by this change.