There have been a number of hints on this front (i.e. adding support for models X, Y, Z), but mostly for the Address Book, to enable Bluetooth dialing and SMS support.
Regarding iSync, things are not necessarily more complex, especially if the phone you are trying to add is similar or identical to one already supported by Mac OS X. Case in point: I was trying to add a SEV600i -- it turns out that iSync 2.1 has support for the K600, but not for my V600i. It took a bit of fiddling around to realize that a precise sequence of steps was required...
- As expected, modify /Applications -> iSync.app -> Contents -> PlugIns -> ApplePhoneConduit.syncdevice -> Contents -> PlugIns -> PhoneModelsSync.phoneplugin -> Contents -> Resources -> MetaClasses.plist
- Add identification strings to the array as shown here. This section used to be a single Sony Ericsson+Sony Ericsson 600, so I turned it into an array to support the 600, V600, and V600i. Some guesswork is probably required for other models, so feel free to add many lines at once; it can't really hurt.
- Save the file; quit iSync.
- Then and only then, add your phone to the Bluetooth device manager. If your phone was registered, then you need to delete it and start again. A good sign is when the first tick-box is not greyed out; i.e. a hint that iSync will work. I had assumed that restarting iSync alone would take the above modification into consideration, but adding the device is what does it.
- Start iSync, add your phone, and sync!

