The problem with disconnecting the battery is that (WM2005 and its new memory model aside) you lose everything, and have to run the Backup program to restore the O/S and applications from the flash card.
In fact I have wondered whether there would be a market for a software product which, at power-up, wiped the PDA and did a restore from flash, then started your specified application. You can get it for PCs; it's called Deep Freeze.
The said product would also disable all buttons, and the touch screen, so they can't be accidentally touched
You are right about screen resolutions - this is why I have put in so much time myself playing around with these things. The screen res of say an HP4700, 640x480, is way better than just about any aviation GPS. Very tempting...
There is an unusual ruggedised PDA with the special polarised display which is totally sunlight readable, but it's not cheap.