I did something like this… wrote it up here:
Feature Request: Virtual Green Room For Shows
I haven’t had a chance to update that post, but at Oliver’s suggestion now use syphon layers instead of NDI. Works fairly well provided you only have a few guests…I think probably the limitation is my hardware (2014 iMac) and organization skill rather than a limitation of the software… have tried a few variations.
It would however be amazing to have this type of feature built in.