Hi Freester,
I don’t think you could use what we use. The reason is too complicated to get into here.
What’s you objection to live updating and ajax?
You are likely using the version of BBPress we use on the grit.cx site.
We don’t have a preview on that. If we did I would likely use jQuery or similar to render a preview. You wouldn’t necessarily need to use ajax as the servers db is not updated until the post is submitted.
Even better would be to use Angular.js to provide live data binding. That would allow you to have the preview updated as you type.
Sketch here:
http://codepen.io/standardspace/pen/raopMr