Is it possible to set up N to do surround mixing if I got a surround panner plugin to insert in 6 mono tracks and then buy an encoder that will convert those 6 mono tracks to a ac3 file ready for DVD?
In a way, yes. If your soundcard supports six or more output channels then you just need to enable at least six outputs. That will usually be like selecting three or four stereo output pairs. When doing playback you can select which output a track goes to. Each enabled output has its own set of aux returns. This will make it possible to pan tracks between output channels that are no of the same pair – normally panning only goes between left and right of the selected output, but by ALSO sending to an aux that goes to a different output pair the effect is that a single track can be played out multiple output channels at the same time, not just the one selected for that track. This is pretty much what 5.1 mixers do since they usually let you pan mono tracks somewhere in the middle of 5 channels and the sub channel. It will obviously be more difficult without a nice pie chart to look at though as is the case in most AC3 encoders.
When a mixdown is done n-Track will produce waves for each output pair. These can be separated into mono tracks and opened in the AC3 encoder. Simply match the channel assignment of each (formerly 3 stereo waves and now 6 mono) track(s).
No all 5.1 soundcards expose the outputs in a way that will allow this to be done, but most multi-IO cards do. For example if you have an Echo Gina (supports 4 stereo pair as outputs) then you’re on your way.