This is research FR, try to use some light server for generating preview instead of using templates.
See diagram of possible variant of work here http://www.jboss.org/community/wiki/VPEcurrentstateandpossibleideastomovingtoo
I have looked into possibility to use light server and have found following problems:
#1 JS execution
Problem: Many of ui components use js for creating it's DOM, in our visual part js is deprecated, so components hasn't be built.
Answer: JS execution not very important, I have disabled it in browser and looks on richfaces demo and icefaces demo sites. UI components renders prety well even without js.
#2 not valid source page
Problem: If source page not valid, it's displays error log, but no one components is displayed. In vpe now displayed even not valid components or components which not completed. So how to rended not valid jsf page on server?
A:?
#3 navigation
Problem: if we will send page to server and get from it resulting html, how to map visual and source nodes?
A: Possibly to set id for source elements.
#4 integration with our current vpe
Problem: it's hard to integrate light server with our current templates.
So I think to use light server for generation not good idea.
Also was an idea to setup a servlet on server and send to servlet a some code and in response gets resulting.
But this works only for components which haven't child, because when component have children it's hard to map source and visual nodes.