Actually, we don't have a "native" QNAP (or other NAS systems) application and I can't tell you if and when will be developed.
We have this on our roadmap, but actually it's not possible to give you a date.
On the other hand, you can use your NAS as a storage for your encrypted files, no problem.
But you will need a computer where the Boxcryptor Application is running.
You can even connect from your mobile devices (via WebDAV) to your QNAP system.
Another possibility is to run a virtual machine on your QNAP (if supported) via the "Virtualization Station" package, then you could install there a Windows operating system and Boxcryptor for instance.