Well, the template seems complicated and greek to me; so here I am sharing my idea
mod_prev
The page greets you with a blank text area, which represents the source/bbcode view, where you can try out any of the default phpbb bbcodes, and hopefully the preview pane, which can be accessed by clicking the preview button should render the bbcode as it is to be displayed.
Any bbcode that the CKEditor based preview pane can't handle can be handled by a slight modification of the CKEditor bbcode plugin.