Can iOS 5 Wi-fi sync be used over VPN?
Not a full answer but as you seem to primarily look for hints and pointers:

According to this MacOSXHints article from three years ago it seems to be possible to route Bonjour over VPN using OpenVPN or LogMeIn. This should (with some hacking and routing magic probably) at least allow WiFi syncing as long as you have computers (not iDevices) on both end of the VPN (and connect the iDevice to one of them).