Is it difficult to save a "rendered" buffer to an image or video file
You can save a rendered PixMap to disk using the SavePixmapPNG command.
Saving on AVI is not that hard, assuming you have a DLL or know how to do it. BM can't do it, nor can it play movies.
If you need to play movies, there's an option for MPEG, with some source posted on Chris C.'s website:
http://www.chris-camacho.com/index.php?option=com_docman&task=cat_view&gid=13&dir=DESC&order=name&limit=5&limitstart=10
won't write though, only play.