Would be a great app.
Also it should do a some more tests than just Video Playback.
I found out that having WLan active at LEAST produces much Heat. io should be tested on internal and SD memory too (at least as an option, having the OC limited by faulty SD Cards doesnt help)
Im not sure if it may create problems. If you are sure it doesnt it doesnt matter of course.

And it should run fully automatic, time doesnt matter then. (i can live with leaving my N900 alone for 10 Hours or so)