I think that would be a useful feature, but I also think it would need some exceptions. For instance, you'd still need to be able to close any boxes that pop up and obscure part of the screen (e.g. warnings).
Buttons are another thing. You'd also still want NRST to work for emergencies. How about bringing up different Map Modes, like EFIS? Also changing instrument layouts.
But I think the idea has merit. Maybe it could be a total lockout as I believe you were suggesting, but some special gesture would put it in/out of that locked mode, like a triple-tap on the screen. Something like that. Or... (this is a serious suggestion): the user could draw an "L" on the screen to lock it, and a "U" to unlock it. That'd be interesting (and probably unique among EFB's). It could also be expanded later to use other letters for other features.