You can't do this without a custom hack
I didn't see a hack on xda-developers ... sorry
From Microsoft's Windows Phone 7 Syncing Guide >>>
... files will automatically sync with your phone wirelessly when the following conditions are met:
Your phone and PC are both connected to your home Wi-Fi network. Note that wireless sync is not supported on workplace Wi-Fi networks.
Your phone battery has at least a medium-level charge and your phone is connected to an AC power adapter (that is, your phone must be connected to the wall charger and not connected to your computer).
You're not interacting with your phone. For example, you're not touching things on the screen, the backlight is off, and music and videos aren't playing (it's OK if a song or video is paused).
It might take up to 15 minutes for wireless sync to start, and there is no way to manually force it to occur.