In this case I would look at a little mixer and route both the onboard and the outs on the DV deck to it. I don't know of any firewire based i/o devices that will allow you to monitor both. There are many options if you go out analog, HDMI, or SDI, but not firewire like the Mojo. Have you set both the Preview and the Export in the playback settings to the 1394 device btw? I have not tried what you are doing but it might work. Otherwise a mixer would be the easiest solution unless you want to export to this DV deck via component or the other analog.
The Realtime Playback and the Export are both set to DV output. What would fix the issue is to have another set of radio buttons under the export option allowing you to have desktop audio during editing and External Device when exporting to tape.
I guess since use of tape is going away it won't matter.... When we upgrade to HD camera's we will likely have SD cards from them and will export to a format our production servers will use..
One of the other things I lose using External Device audio looping to the speakers off of the RCA jacks on the DSR-25, I can't use the great volume keys on the Premiere Pro keyboard. They track to the windows sound control.
The Recording to the timeline is not and issue as the mute timeline audio setting kills it while laying down tracks. Oh well can't win them all.
Any other idea's....
The audio mixer would be the option then. That would give you the ability to monitor regardless of what you are doing between multiple outputs.