We're working on supporting OMP fully through a new script render job node (which is itself a script node) that will be turned on by or before 3.1. That is going to be the officially supported path once it is out, and will expose all you are asking about and more.
In the meantime, if you just want to play back .mp4 video layers over a stereo cube map BG, I think there is some sample lua/JSON template code on the forums that can get you started. The ORBX Media render job node will generate correct JSON+lua as part of the render job. This will cover adding in audio, video layers, glTF (exported from mesh nodes) and other elements to be bundled and played back in the ORBX media package export.
As shown here, we are working on supporting more than just MP4 files (e.g webVR, Unity games) inside ORBX media layers:
https://youtu.be/RE9GwyzL9MU?t=103In the next update you'll be able to launch an ORBX URL from within the Samsung Internet Browser, and no longer need a native app (e.g. whitelisting OMP) or even the ORBX media folder sideloading system to deploy your ORBX to Gear VR (and soon Daydream/Tango -and PC WebVR).