Forum OpenACS Q&A: What stops us from using OO?
Therefore I think the easiest way to test if supporting OO will attract more users to OpenACS would be to make sure you expose the TCL API and the ACS Objects to Java and allow the Request Processor to serve pages written in Java along the ones we currently have in TCL. This allows anyone to use the existing functionalities while enabling her to write her own additional functionality in Java. This approach has the benefit of easier connectivity to proprietary third party tools, as they usually tend to come with a Java API.
Sadly I have no clue how much effort this would be and I guess unless someone really needs this for a client project or because of the need to finish a thesis we are not going to see much process.