weather talent typically look at an offscreen monitor and can see themselves over the graphic (they also use the same maps and graphics every day and know what they will be over).
with out a hardware keyer, you might try using sheets of acetate and dry erase markers to create overlays to put over a monitor that shows the output of the camera camera. you'd could draw the points they needed to interact with on the acetate and and they'd be able to see themselves and the drawn points to be able to know where to point to,,,
[jolyon hennings]"The presenters are corporates, so have not used green screen before..."
Well, then there's going to be something of a learning curve for them.
I like Kevin's suggestions. I too work at a TV station, and the clear overlays on monitors is an excellent idea.
They'll need time to practice: if you go with this trick, know that everything is OPPOSITE of what they see in a mirror. When you point to the right in a mirror, you see yourself pointing to the right. When you point to your right as a camera takes your picture, the monitor image show you pointing to the left side of the screen.
Weather people do it every day, but they've had hours of practice. If your folks have neither the time nor the patience for this simple truth, you might want to consider a Plan B.
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA