Right, I am assuming a soft keyboard as part of the UI here. But yes, eventually hardware keyboard is important.
> I'd (re)write the whole UI, designing it from scratch to suit small screen
This only needs to change if your screen is small -- otherwise the sage notebook seems reasonable at tablet sizes... but yes, if you're aiming at the Ti hand-held form factor it must be changed.
> I'm a _______, so I'm probably underestimating the work involved here
C'est la vie
> That's a x86 computer running MS-DOS fitting on your palm. From the mid-90's. I think we can do something at least as cool today.
Definitely. This project sounds awesome (and also a bunch of work) -- good luck!