Maybe an odd question, but think 1990's desktop software that supported embedded macros and wizards, written by users to extend/enhance out-of-the-box functionality...
B4J supports inline Java code, to perform some things that B4J can't. {Albeit I've personally never used this powerful feature, to-date at least.}
Yet I've not seen any forum threads that ask whether B4J code, written by B4J application users (i.e. not by developers, like B4J libraries and custom views are), can be interpreted or compiled on-the-fly and yield UI controls -- and trigger their related event code too (again, as written by users) -- added to a specified pane in the [parent] B4J application?
I assume the answer is "No". If so, then alternatively could a Java code file written by a B4J application user be dynamically loaded from a non-app directory (and compiled in memory) to accomplish the same?
If that's a second "No", then can a B4J-generated .jar file be dynamically called and executed from a parent B4J application (and visually attach itself to a specified [child] pane displayed within the parent application)?