And the answer is that it is possible to create .phoneplugin plug-ins inside /Library/PhonePlugins, using the same layout found inside iSync. The plugin, let's say it's called MyPlugin.phoneplugin, has to live in /Library/PhonePlugins, and needs to hold the following files and folder structure:
/Library
PhonePlugins/
MyPlugin.phoneplugin/
Contents/
Info.plist
Resources/
MetaClasses.plist -- containing the stuff to support my phone
XXX.tiff -- my phone's icon
The question is: what will happen once Apple adds support for my phone? Won't there be any conflict?
[robg adds: I haven't tested this one, and this hint assumes some level of knowledge of modifying plist files, as well as the structure of the Info.plist file, etc.]

