I'm not sure if this has been brought up before, but I had the idea when we were talking about the spectator button. Sorry for the crappy mock-up, I made it with MS Paint :)

This might take a bit to pull off. I haven't thought of any exactly intuitive ways to make the control locks work yet, but I'm thinking...

Also, gonna have to learn how part selection is done, but I'm really, really hoping that will be trivial.

To explain the image a bit:
First you have the players who are spectating the vessel; need some way to select them - probably by making the names buttons.
The thing in the red circle is a button to hide the menu below that point.
Give Master Control gives the selected spectator complete control of the vessel.
Selected Parts are parts you've selected with the mouse, normal KSP style.
Action Group Control are buttons that would stay depressed if selected, so you can tell who has what action group.

Notes:
There should probably be only one spectator to a part, unless someone can come up with an intuitive way to share functionality.
Also need a box/list for what vessel parts a spectator currently controls.

tl;dr: 👍 for godarklaser