Ok... first I was wondering how a flash worked so I took a look at this: http://electronics.howstuffworks.com/camera-flash.htm
and it is very well explained there. In short a standard camera flash in most hobby cameras are xenon strobes. I also tried to look at relative efficiency of a xenon strobe flash vs LED strobe units. I was pretty unsucessfull at finding a comparison in a very timely manner. In fact I could not really find a lot of stuff on xenon strobe efficiency. Given I was not going to spend a lot of time searchin online. I did note that LED efficency was in the range of 90%. If I were going to bet I would say that an LED "flash" would give better efficiency. I would also have to make a bet that a xenon strobe is brigher.From personal experiance with a specialized seperate camera flash and built in camera flash units in a hobby type camera I would say they are battery hogs. I have not had experiance with a dedicated camera "flash" using LED's but with what experiance I have had LED's are much easier on batteries.
I have looked at many (most) of the rigs in the forum and there are similarities. The commonalities I have found are that they are using overhead light of some sort and the platen (glass or plastic) is set at 45 degrees to that light (notable exceptions do occur) and the camera is rigged parallel to the platen a little out of the light stream. This is set up this way for a couple of reasons. The camera really needs to be parallel to the surface of the platen/ book to avoid optical distortion/ keystoning if for no other reason than just to make things simpler for post processing. Second the overhead light avoids glare on the platen. If one puts the light directly behind the camera the result would be sort of like looking at a plate glass window as a mirror. Lots of glare and bounce back of light.
Book scanning using a camera flash. I think, based on the above, using the built in flash would cause lots of problems with glare. A seperate camera flash set up to be in sync with a camera, hot shoe on an extention, placed in the overhead position or some other appropriate place could work very well to take these type of pictures if one can avoid glare. Generally a seperate flash is expensive and it must be paired with a camera that is able to use one (can also be expensive). This is all a bit acedemic if one already owns a flash set up. Something might be done with bouncing a camera flash around to make the light more diffuse (often done by photographers in a studio) but it would probably play hob with camera exposure if it was done from a built in flash. I expect that the cost would also be lots of batteries or a specialized charger for a seperate camera flash.
Compared with the various rigs that Daniel has made with LED's the camera flash would be pretty inefficient. The most recent LED rig that Daniel has been working on looks like it should be fairly simplified and light on battery use. I understand that the documentation is difficult to follow on the eirlier versions but the newest one looks pretty straight forward and I expect it will have pretty stand up documentation when Daniel is ready to release a diagram of his build.
Thats my take on the camera flash option.