DarkSable :
Actually, this could be super nice for productivity tasks; being able to type a lot and still move the cursor without having to keep the keyboard off-center is pretty sweet.
Nah, it's throwing out the productivity baby with the bathwater. It's asking you to use a non-ideal mouse-thing that will absolutely certainly be slower than using a regular mouse to try and save a third of a second moving from mouse to keyboard. You'll get the best productivity by having the best device for two very different, very specific tasks.
Whatever you lose shifting from one to the other, you gain from having a proper scroll wheel (for example), sensitivity that perfectly suits your tastes and a potential bevy of extra buttons to use. For cursor movement you have arrow keys and the page up, page down, home, end combo to work with. You can get around a document well enough even just using the keyboard. It's notable that people with totally reconfigurable macro-bereft keyboards still typically don't set them up so they can keep their fingers on the home row.
The real bottle neck in productivity isn't how much you can physically type, or how quickly you can dance around the document, it's having sensible things to write when you get there. Absolutely no-one just sits at their keyboard and types at 100wpm end to end, bemoaning the fact they had to slow down to open a new document.