Talking about how others do it, do you know how many bounces Unreal4 uses? Yesterday night I finally got it working with one bounce, at a stunning 5 frames per second (but on an a 2009 videocard, and a lot of other stuff going on). It looked pretty cool, apart from the blockiness that also appears in GI more or less. But the real problem is that, of course, a single bounce still isn't much to realistically spread the light. I believe the VCT video shows 2 bounces at some point. Would that mean I have to:add up the indirect light and redo the whole thing? Ifso, it's still not correct as we only got Indirect light for geometry that appears on the screen.
The 5 FPS that you're getting are for glossy reflections or diffuse GI? I'm particularly curious about this because from my experiments it seems that tracing an octree is much slower than tracing a simple 3D texture which is why I've dropped them. If I recall correctly, on my Nvidia GTX260 I got about 5 FPS tracing an octree and 30 FPS or more tracing a 3D texture.
I'm almost sure UE4 uses a single bounce for sharp/glossy reflections as well as for GI which is more than enough for most cases. Notice that in the case of reflections you would only need more than one bounce if you had mirror like surfaces bouncing light from each other which is not that common and the impact on image quality is not very noticeable. For the diffuse GI, more bounces would increase the image quality a bit but a single bounce already provides pretty good results.
Including the indirect lighting into the voxel volume should work well. For that you'd need two voxel volumes, one that represents the scene voxelized with direct lighting only (this is what you have now, right?) and use it to generate a second volume where the scene is voxelized using both direct lighting and indirect lighting (calculated by cone tracing the first volume).The cool thing about this approach is that you don't have to build an Octree for each volume, since the scene structure remains the same you can keep a single Octree and use it to index the two brick volumes.