I've been using PKL 0.3 for the last few weeks and the 'extend' key is incredible. For those who don't know about it, you simply hold down the Caps Lock key and you have a new modifier key. If you press it once, you get the usual single backspace. Here is a picture from the PKL download showing the default keys in Extend mode:
Does anyone know if such a thing is possible for Linux/BSD? I'm happy to look into it, but I don't really know where to start.
Edit: I've started looking into it. XKB is a frightening beast!
"It is an undoubted truth, that the less one has to do, the less time one finds to do it in." - Earl of Chesterfield