I already have a content lister in Java, its what I boot into directly when I do my Java boot test: cuts the iLiad boot time nearly in half.
I'd love to replace all the heavy weight Matchbox UI stuff with nice light weight threaded Java widgets but iRex has the Java restricted from drawing into the Matchbox UI area on the screen.
As for doing a straight up slow boot contentLister replacement: the Java ability to execute native applications always returns IO Error when you invoke it.