With a quick glance, I'm not entirely sure if this is possible with the calibre plugin API.
My idea for a plugin is one that allows Calibre to synchronise with books bought for a Kindle on the Amazon store. The implementation could be done by spoofing the service and registering Calibre as Kindle for PC and then accessing online storage. This would also prevent the download of personal documents as they would most likely already be managed by Calibre.
This, in combination with
pwr's Kindle proxy would allow Calibre to be a unified location for ebooks on the Kindle.