I'm currently doing proof-of-concept testing on a WinXP tablet with a 12.1" screen (the aforementioned Motion Computing M1400), so basically, I'm doing that same thing but with the bonus of having live editing available by having Word installed on the tablet.

And yes, I once upon a time seem to recall seeing a web site for a company that had a purpose-built tablet for sheet music (pre-ipad days), so it might have been them.

Interestingly, a google search for those people turns up these people, who have a pre-made list of computers, some of which look like the sorts of things I'm looking for:
http://www.musicreader.net/hardware/tablet-pc/tablet-pc-comparison.html

I'm going to investigate the 13" ones in that list and see if I want to get one of them on ebay or something. (*EDIT:* Never mind, all the 13" ones on that list are 16:10 widescreen displays, we're back to my same original problem.)