jomz/radiant-page-preview-extension — comment on commit a804ff9da8b4a071ad0653c9e01bada1df9105ba
File:  line 0
By jomz:

I think that if your radiant version has bundler support, it also has preview functionality.
See https://github.com/radiant/radiant/blob/master/app/controllers/admin/pages_controller.rb#L32