Can applications predinstalled in system/priv-app be updated via Play Store?
If the "pre-installed app" uses the same signature as the one on Google Play, it of course can be updated that way – same as e.g. Google Maps and other pre-installed stuff is.

Note that, however, those updates would not be integrated in the ROM, but go into user-space – "overlaying" the pre-installed version. This gives users the advantage of being able to "uninstall updates", but at the cost of "double space" (the original APK remains in /system – not that they could use that space otherwise, though).