I also don't think it's pointless. I would really be interested to revive one of my dead N900, just as a tablet pc, it would be really awesome.

But... HTC HD2 have a built in Windows bootloader (N900 have a Linux one), also the CPU in the HD2 is more powerfull. I don't think you can have a good W8 experience on that low hardware (Intel Atom N270 1.6GHz runs W8 pretty rough...) also you have to think about the GPU... W8 requires some visual memory to run animations smooth...