Update, Friday June 20 2014: Onda has reversed course: Onda Re-Announces V819i Windows Upgrade, Onda To Release V975i Windows Upgrade.
Thanks to malkovic in Comments who provided two links to very unofficial Windows upgrades for the Onda V819i and the Onda V975i.
V819i install Win8.1 System Tutorial [Google Translate]
V975i install Windows 8.1 UP1 [Google Translate]
Both forum threads have locks on them, requiring registration (and probably replying to the thread) to view the hidden content (which consists of instructions and likely file download links).
I provide these only as information.
The risk of bricking your tablet is very high. Neither of these look to be finished products (the one for the V975i requires using a bit from the Teclast X98 3G Windows upgrade!).
Previously here:
Onda’s Windows Upgrade Cancellation: It Gets Weirder
Onda Cancels Their Android Tablet Windows Upgrade!