Do it in Reaper. 11 instances of Nebula as an FX chain. Set track to 12 channels. Now modify the plugin routings: 1st instance: In 1 to In L, no input to In R, Out L to Out 1. 2nd instance: In 2 to In L, no input to In R, Out L to Out 2. etc. etc.
Now render track to stem track, see if the resulting file is actually 12-channel (12th channel remains empty for obvious reasons ) If that doesn't work, do a normal bounce to 12-channel-Wav-file - should do the trick in any case.