I’ve thought about this a little bit, certainly there are better minds than mine that will ultimately figure it out. But here are some of my thoughts:
[Update: 18 May 2009 - The Icom Repeaters will not handle the proposed "narrow" DD without a firmware change and I have no indication that Icom has plans for the same. However, other repeater controller projects are in the works and at least one experimenter has tried "narrow" DD (6.25kHz.) on the same channel as DV and found that eventhough the Icom radios do not know what to do with the DD traffic, it doesn't harm them either.]
You have a “DD” only radio, 6.25kHz. GMSK. Lower complexity, parts count, to build — would make a neat kit. If it was operating on a channel that had mixed DV/DD traffic, it would watch for packets that contained DV and would back-off to allow the higher priority (real time) DV traffic to take precedence. The DV users would hear nothing on the channel when DD traffic was being passed, as there would be no voice for the AMBE2020 chip to decode. When the channel was free the DV user could initiate a transmission, which would tell the DD radio to take a less aggressive approach to channel usage. When a certain amount of time had passed without any DV traffic on the channel then the DD radio could take a more aggressive approach to channel usage.
A dual-mode (DV/DD) radio would implement the same protocol for DD traffic, since the channel may have other DV traffic, not initiated by itself (e.g. no PTT event). DV radios would also listen for DD traffic on the channel and set a flag, so that if a user goes to transmit and gets inhibited by (DD) traffic on the channel, then it would automatically insert a DV packet at the first opportunity to let DD radios know that there was higher priority DV traffic ready for the channel. (The DV radio, would signal the user when it had sent that packet so that user would then know they could PTT.)
I am not aware of a specification on this.
I would venture in many EMCOMM situations the accuracy and routability of DD traffic (it has full TCP/IP capability) would give it more utility than DV traffic so a repeater asset that is normally used more for DV on a day-to-day basis would see a much higher percentage of DD traffic during a managed incident.