You can still use Nebula on multichannel mixes you just need to split your tracks out into buses rather than processing through a 6+ channel master. Those using hosts that don't easily handle multichannel mixes have to do that anyway.
You can definitely use Nebula in surround applications. Need a surround mastering compressor ? Take 3 Nebula instances with the same compressor program open with identical settings (sidechain compatible program). Route all signals to a buss called "Sidechain" (can be stereo) Put the nebulas on the 3 busses (Front LR, Surround LR, LFE+Center). Have all these instances receive its sidechain from the "Sidechain" bus - voila ! They will react identically. Explosion in the LFE will lower all other channels etc.