Setting renderer merge-fonts from code

Hi.

I'm sure I'm missing something simple, but I'm looking to set the "merge-fonts" configuration via code on the FOUserAgent.  And I'm failing.

I've found how to read it, but there's no setters:

            PDFRendererConfig rcfg = (PDFRendererConfig)userAgent.getRendererConfig( "application/pdf", new PDFRendererConfig.PDFRendererConfigParser() );
            PDFRendererOptionsConfig rocfg = rcfg.getConfigOptions();
            rocfg.getMergeFontsEnabled();

Does anyone know the magic?

Thanks
Mark