Many line tools have the option "start--middle--end". I'm not a fan of selecting stuff with a mouse in menus. Couldn't the arrow keys be used to set the reference point? For example: if I'm using the vertical line tool, pressing the right arrow would move the reference point from "start" to "middle". And the next press or the right arrow would move the reference point to "end". The same logic would apply the other way around with the left arrow.
regards: Seppo Silaste