Wow, you actually read it? Cool! : )
I tried something similar to what you say, working with my cousin. We'd use a different yet simple easel and a stand for his Nexus 10 (hi-res big screen to read on, takes good pictures as well), to try and keep it cheap, light and portable (except obviously for the Nexus part but that's his problem). It was actually a bit faster than my setup (but not by much, as I say further on in that thread) but we gave up in the end. Our problems with it?
-Setup portability is pretty much gone as soon as external lighting has to be used/taken into account. More stuff to carry around, much less places where you can work.
-The pictures weren't good enough by comparison. Think I've become used to a steady resolution of 600x600 dpi (surface-contact scan), a must for small print and figures with fine detail, frequent in the kind of books we scan.
-No matter how well you align everything, non-trivial post-processing was almost a necessity for two reasons:
1. De-warping: many books won't stay perfectly flat (which is the why of the platen in canonical designs)*
2. Resolution is hardly constant over a set of pages, let alone across scans in different places. More software fiddling... yaaaargh!
*Wiping a scanner over the pages surface eliminates 99% of any remaining problems with the side that is 'naturally' flat (by gravity) anyway in my L+clamp setup.
So, is it possible? Yes, and we did it. We just deemed it an inferior solution (my cousin purchased a ruler scanner, yeah!). YMMV ; )
However, you got me thinking again, one could add an upper 'fork' to my 'L+clamp' design to work with tablets and phones. Note I'm against non-vertical/horizontal stands for books -- unless you're working on both pages at once (and can efficiently flatten them with little effort -- back to canonical designs with platens) there's no reason to prevent gravity and/or walls from helping you as much as possible on the side you intend to work on. Similarly, why create a necessity to fix things into place just to get particular orientations, as opposed to do it simply to keep stuff from moving around/dropping?
The book would lay open in my L+clamp, and you'd have a 'fork' over it -- two simple but somewhat adjustable arms would stem from both ends of the upper side of the L and your 'camera' device and lighting would lay on or be fixed either directly to them or a simple platform (keep thinking flat or foldable pieces of plywood/cardboard and elastic bands). You'd be turning pages manually almost non-stop this time (no physical obstacles, no scanner wiping, just an open book and a hand) and you could even use some transparent plexiglas sheet to further flatten the horizontal pages if necessary with little slowing down (glare problems loom in the horizon). I'm sure programming such devices to take pictures with pre-set settings every X seconds would be fairly easy, so I see interesting possibilities here for super-phone/tablet owners.
I can think of a possible show-stopper, though: can YOUR devices be used/tweaked to take good pictures from a short distance (think about 40cm)? This is not an usual need and these are not exactly cheap so buying some just for testing is almost certainly not an option. I'm perfectly happy with my 'crappy' phone, so I'm not buying one even to give it some real use...
Other than that I see many advantages:
-As good as my design in every regard except size, but not by a big margin there.
-Scan becomes almost as fast as turning pages of the book to lay flat. Further approaching the physical limit of attended scans. : )
-Little power needed to efficiently light from a short distance. Leds, batteries, and whatnot (got a good phone/tablet flash?) become a real possibility.
Feel free to use or improve any ideas here, as long as you report back ; )