I finally managed to get the code to synchronize an Access database with a SQL Mobile one. There are still hundreds of little issues to solve like retrieving error information from the custom Access provider and making sure the IDENTITY values are correctly recorded. But it is working!
If you want to take a look at how the code is coming, download the latest version of DesktopSqlCe (1.3.200) and the DataPortSync pre-release 1.0.200. Be prepared for a bit of a rough ride!