MP900C: Move My Documents to Internal Disk or Storage Card

I've been putzing with this for the past hour or so with no luck. I've got an MP900C (actually an MP900 with the official CE.NET 4.2 ROM), and I'm trying to move My Documents to either Internal Disk or Storage Card. I can update the ShellFolders registry key just fine, and it defaults to whatever path I give it just as it should, but ActiveSync is being a pain in the rear. (What's new, right?) I'm running Windows XP with ActiveSync 4.5, and going back to an older version isn't really an option as I need to sync with some WM5 and WM6 devices as well.

I've found the FileSyncPath and NoSubFolderIn values in HKLM\SOFTWARE\Microsoft\Windows CE Services, and tried changing them from "\My Documents" to "\Internal Disk\My Documents" and "\Storage Card\My Documents", but the end result is always that ActiveSync wipes the PC's local sync directory, and then any files I put back in there fail to sync back to the MobilePro with a bunch of bizarre errors like it can't access the filesystem or something. I also tried editing DeviceSyncFolder in HKCU\Software\Microsoft\Windows CE Services\Partners\[partner ID]\Services\Synchronization on the desktop, which doesn't have any noticeable effect.

The other thing I tried was to leave all the sync paths as "\My Documents", and setting the internal flash disk to mount as "\My Documents", but then ActiveSync gets really confused and thinks the MobilePro doesn't even have file sync capabilities.

Has anybody successfully moved the My Documents folder on CE.NET 4.2 without breaking synchronization?