Sadly alternative UI automation libraries tend to be python based as well, but maybe they might run under Java/Jython. You might want to see if Sikuli, AutoPy, PyAutoGUI, etc. will work for you via Jython in Java version of RF.
You could also consider automation interfaces that might not need python that hopefully is portable that you can run on the system under test and interface to it with the Java based RF package you have via Selenium2Library via the RemoteWebDriver option (or the AppiumLibrary equivalents).