I had something similar in mind when I was still dreaming of the rumored N900 being a real tablet. A proper mod with a gamepad split in half and the phone in the middle would be cooler and less top heavy, though.
Yup, I can very well imagine how this setup would tire hands quickly. Maybe attach a camera tripod underneath to ease the burden lol
Sheesh, I am still hacking around on my n900 to get my third party sixaxis to work over bluetooth. Shame I don't have an original one for troubleshooting, have to annoy a few of my friends and borrow one