Cool! 
Thanks!
Mavlink2rest can communicate both ways, but at the moment it only displays what it receives from the autopilot. I just had a meeting with the software team and brought this up, and they agreed it would be useful to also capture and display messages it receives from clients (for this kind of thing, and several other purposes), so it’s something we’ll try to make possible.
That said, Companion 0 is only receiving critical updates at this point, so I expect the feature will only end up in Companion 1 (which is currently available in Beta).