I think it is a good idea, and the concept is very simple to do, but I can't see what it would effectively do.
If it just stops and starts beeping at you, that is bad because it will ruin the part at the point it stops.
If it pauses the print and retracts to a safe position waiting for more filament to be loaded, it might then be difficult to restart and get things to bond together because the top layer will cool down (unless you were sitting right there to do it immediately like for a manual filament color change).
If it just aborts the print, what's the point of it other than perhaps saving energy?
I strongly suspect that I am missing something here.
Can anyone explain to me how such a thing would effectively function with our printers?