So as a follow up, I contacted Sync support. Here is the transcript:
Chat InformationPlease wait for a Sync Agent to respond.
Chat InformationThank you for contacting the Ford & Lincoln-Mercury Sync Support Center.
You are now chatting with 'Adam'
How may I assist you?
you: I have a T-mobile G1
you: and am able to use all features but text messaging
you: and wonder why that is
Adam: Unfortunately, few phones support the ability to forward text messages over Bluetooth, as is required to use that feature.
you: is there an sdk for sync?
Adam: Though T-Mobile allows that ability to be used on their network, it is unlikely that the messaging application on the G1 can be changed to send the messages over Bluetooth +AT commands. Those commands are how SYNC communicates with the phone for all features.
Adam: No, there is not an SDK available.
you: the +AT commands, are they lower level commands within the OS, or could an app be built (as it is open source) to do such.
Adam: The last time I had asked about the issue, I was told that there were no plans to make an SDK available in the future, either.
Adam: The +AT commands are part of the standard Bluetooth protocols.
Adam: It would be possible for an application to be built to send the messages, though I don't know about intercepting the messages from the typical G1 texting application.
Adam: You would also need to be sure that the feature is told to SYNC during the Service Discovery phase of the connection, so SYNC knows that the phone can be capable of that. I am not sure exactly what would need to be changed, but I believe that information is in the GSM specification for text messages.
you: are there any other elements that would need to be involved?
Adam: I am not aware of anything else, though I am not a SYNC engineer.
you: fair enough
Adam: Is there anything else I can assist you with today?
you: well then thank you very much for the information, and hopefully we can make something work.
Going to post this to the dev forum and see whats possible.