Ok, the 4.3 OTA update is out now for a few months, but I didn't find the time to do the update on my rooted Nexus 7 device.
Another reason for the delay was the fact, that the Nexus 7 Toolkit wasn't released until a few weeks ago. My Nexus 7 already showed me the information that a new update was downloaded and ready to install.
Well, finally I decided to do the install (making a full Nandroid backup prior to the update), but when hitting the button "Reboot and Install" nothing happened.
Usually after waiting 10 seconds the device should reboot and install the update, but the reboot did not occur. My battery wasn't the problem, because it was almost fully charged.
First thing coming to my mind was to flash back the stock recovery. Unfortunately that didn't solve the problem.
The next thing I did was do hit the button "Delete data" of the app "Google Service Framework". This usually forces the system to reset the update state and check for newly available system updates.
And this did the trick. Clicking on "System Updates" the device told me that it was uptodate with a last check date of January 1st, 1970. So I manually hit the button to check for new updates and my device started to download a fresh copy of the 4.3 stock rom.
Now hitting the button "Reboot and Install" rebooted the device and installed the Android 4.3 update.