(i made this picture in the Scratch forums because i don't know how to make dropdowns in custom blocks)
Wouldn't it be cool to be able to detect left clicking, right clicking, and middle button clicking?
I'm not sure why you would need middle button clicking, but it wouldn't make sense to only have right and left click and not middle click.
So, because right clicking in the editor actually serves a purpose, it should only detect right clicking in the project page or in fullscreen.
It would be useful for me in my missile command program as it has 3 missile bases 
But it would make development/debugging of all but a trivial program quite difficult because of all the swapping back and forth between editor and full-screen
But full marks for thinking outside of the box
But maybe it would be possible for one of the javascript whizzers to come up with blocks that would work in full-screen
We're not going to introduce features that work differently in edit mode from in presentation mode; that would confuse users, make debugging hard, and lead to a lot of spurious bug reports. So you're definitely not going to get a right-button block.
Middle-button isn't out of the question, although projects using it won't be very usable on tablets or phones. People have asked for this before, but we think it's a low priority issue. It's a JavaScript one-liner, so you can implement it yourself in a project using the JS Function block.