Is there any other solution for the features JTextPane RTF does not support?
There is a commercially-available RTF converter from Novosoft which is callable from Java and can convert RTF (including embedded graphics and tables) to the XML XSL FO format. This package references converters which can transform XSL FO to HTML, PDF, Postscript, and other formats. You may find it advantageous to do a one-time conversion or even conversion on-the-fly for your RTF content.