I actually looked in to the code for this a while back to see how they forward notifications. It's not a back channel. It's just they added an additional notification channel for Android System, so Android System basically sends the original notification to the destination profile, then another to the active profile.
Is what you proposed possible? Of course. Anything is possible when you can modify the source code at will. Is it possible to achieve without poking holes in profiles' security? Maybe.
I do know that the current setup cannot be changed slightly to send over short bits of text for copying and pasting because you can't copy from notifications (that I know of).
And since the thread is about this, no, sending files this way won't work. If they did do that, the file size would be very limited, something like <5 MB.