The Field Fallback Module is probably your best bet for this. We are using Glass Mapper and Field Fallback (Sean Kearney's module) to provide language fallback. We ran into issues getting lateral fallback working (w/MVC, can't speak to a web forms implementation). Ancestor fallback should do exactly what you are looking for.
I'm not aware of any way to get Glass to do this automatically. I have a few thread on the Glass Google Group, here is one related to field fallback: https://groups.google.com/forum/#!topic/glasssitecoremapper/-uUsb229LzM
Hope this helps.