GWT Reflection appeared to have GPL licence and thus cannot be incorporated into Drools code base :(
I've found another library which has the support for introspection/reflection of Java beans - Gwittir, which comes with LGPL, and thus can only be included as a jar, but actually it doesn't provide the functionality that could provide us the features for plugging the content editors via Java Reflection API. So anyway we need to extend it.
I think I better go my own path and re-implement the code generation myself. There are 2 reasons for that: I will have the better understanding of the mechanisms behind GWT, and also improve my commit rate for the project :)