Sorry - I had a jira task on this a while ago but I'm still trying to get geoserver to build (and I've been busy).
You need add code like this in the code that sets up the graphic the render draws on:
BufferedImage image = new BufferedImage(width, height,
BufferedImage.TYPE_INT_ARGB);
LOGGER.fine("setting up renderer");
java.awt.Graphics g = image.getGraphics();
if (!transparent) {
g.setColor(bgcolor);
g.fillRect(0, 0, width, height);
}else{
int type = AlphaComposite.SRC_OVER;
((Graphics2D)g).setComposite(AlphaComposite.getInstance(type, 0));
}
renderer = new LiteRenderer(map);