Sorry for raking up such an old topic, but I was experiencing this as well and perhaps I can ask if things have changed since then?
I do also hypothesise that WMP does read the library file in the MSMETADATA folder, which is a consolidated database for the device. If that file can be accessed by MC, and the format reverse engineered (is this legally allowed?) or if it is some open format, then we can do syncs of such metadata.
Alternatively, a plugin can be written to update the JRInfo.xml file, if one is using a different media player (such as Conduits) on the mobile device. Does the JRInfo.xml file contain sufficient information for external applications to write to it or if its schema has provisions for storing the said metadata? If MC's devs can confirm that this approach is plausible, and if time permits (difficult currently) I may get my hands dirty to write a plugin (for my mobile device) which does this. Ratings and play counts are useful statistics to see which files should be synced to a portable device as I probably may not want to carry my full media library with me (only those I listen to most often, for example.)
Having said that, WinMo users are a dying breed...