The External Audio Effect allows you to specify speakers and you can use any number of these.
How would you do that? As far as I see, I can only create a row of effects, with the returns going back into the same track after the External Effect device. This could work for live input (be choosing the original input as return), but would not work for synthesized sounds or tracks, unless I patch a hardware cable to reroute my sounds...
I put eight parallel externals into a rack, and map the gain to the eight macro knobs, automating these from my panning device. I then place one instance of this rack into each track that will go to the speakers. You are correct, the signal chain stops with these devices - they are the last device in each track's chain.
Not perfect, but better than a poke in the eye. You can accomplish a variety of routing solutions, and you can also write reasonable spatialization code.
This is a very nice approach: I like the External Effects idea (not as much as I would like to have multiple outs from M4L devices, but this is a smart work-around).
One feature I want out of a spatialization panner is the ability to route the Left and Right signals from a stereo track to different places in the spatial field. If one were to take a stereo track and use the method you mention above, then the stereo would be summed to mono as it is routed out of each of these External Effects (you have each External Effect's "Output To" parameter set to one mono output which is presumably routed to one speaker. So it becomes mono. That's ideal for some situations.
But If you would like to insert this panning system onto a stereo track, and have individual control of the panning on the left and right, then you could double the number of parallel chains you are using (Let's assume you are building an 8 channel panner): You would need 16 chains, each with an External device on it. Now insert a Utility plugin before each one of the External Effect devices. Set the Mode setting in half of them to "Left" and half of them to "Right". Now you've effectively set up output routing for 8 Left channels and 8 Right channels. Of course, you now have to create two panning control systems, and map one of them to control the Left and one control the Right....
Just a thought in case anyone other than me wants to pan Left and Right portions of a stereo signal separately.
Edit: Aaargh - I hadn't thought about the fact that devices inserted on an audio track are inserted before the track's volume fader. So, we won't be able to control the overall level of a stereo pair being routed to our "surround" speakers using the Track Volume. Maybe the value of the Track Volume can be used as a control for volume scaling in the surround panner itself. Again - thinking aloud here....