I don't understand how it could be more tiring than tapping with the thumb? Unless your touchpad is rough or requires too much force.
Many of us make similar motions all day long on game thumb-joysticks.
Why not simply switch finger every now and then? It takes a fraction of a second and fits perfect into the "release finger to insert space"-routine.
the way its implemented, thumb is only convenient to type using the keyboard.. and if you're right handed, its extremely difficult to master it with left thumb..
Its like learning to write with both hands if you want to switch thumb!
Try this one: http://ddotdash.com (a haha-only-serious prototype of a Morse Code keyboard)
Take a look at iDitDahText: http://iditdahtext.com/iDitDahText.html
I used this for input when I had an iPhone and it worked surprisingly well. There's also a similar android input method but not quite as well executed as this one.
I just did a small experiment.. I took the last text I sent from my phone (about 300 characters) and tried pseudo-typing it according to the keyboard the video explains.. and man, my thumb hurts! I would really prefer if there was a way to input text with multiple fingers without slowing down.
wonder how tiring it would be to type everything with this method.