Syncing Sideloaded books to K4PC/Win8

[DRMMR76]

It seems a month or so ago, Amazon disabled the ability to sync non-Amazon ebooks to the Kindle for PC or Windows 8 apps. I can still sideload books onto my Paperwhite, and have them sync to the Kindle app for Android as a Personal Document. But so far there does not seem to be any way to enable syncing with any PC application, either the K4PC program, or the Windows 8 Metro app. Only books purchased from Amazon show up in those apps. They've even greyed out the PC and Win8 apps in the Manage My Kindle/Send to Kindle setting on Amazon.com. Only actual Kindle devices like the Paperwhite, and the Android apps are supported. Anything running on Windows has been totally disabled.

Has anyone figured out a workaround to this yet? I have about 1,000 books that I loaded via USB onto my Paperwhite, none of which are from Amazon. I would like to be able to leave a book on the Paperwhite, and continue it on my Windows 8 PC or tablet with syncing. Is there a way to make this work now that Amazon appears to have gimped it from the native functionality?