My first thought is this would be a terrible idea, since it would be non-trivial for R to know which package you want a specific function to come from. But, since the forum was down for a day the idea had some time to percolate in my brain.
I could see this as an interesting (and perhaps useful to some) idea if it were assigned to a new keybinding like Shift+Tab.
In the meantime, there is a useful package: prefixer
The package includes an add-in with a Shiny widget which will go through an active file and let you prefix or not prefix things as you go.