Yes, I read that thread and can see that the groovy libs are different between the products. That knowledge does not help resolve my problem on my system. Furthermore, that thread is referencing a newer version of the products than the one I am using.
The thread closest to my question is the one titled "LoadUI custom plugin: JVM and classloader", to which I posted a question asking if the OP ever got it to work on their system. Nobody responded to my question on that thread, so I started this thread. So far, I have not found any information on this forum that describes how this can be made to work.