That is true, ko ko, that the idea was mostly just thrown away. And in fact I've wondered if the level of image/emotion projection that Elfangor used in The Invasion was a KASU, like so much else in that book. But, nothing ever happened to directly contradict it, so in the end I figured there's no real reason not to accept it as canon.
As far as how much concentration it takes to morph, I had been under the impression that it does take some, but not all
that much. In Megamorphs #1, Rachel was able to morph without even having any memory of her ability to morph, basically just by wondering if she was really human. Not to mention the numerous times that they have morphed despite being very nearly unconscious (or completely unconscious, as seen in book #6). It would probably take a good deal more concentration to project images into someone's head than to morph on your own, of course, but I don't think the level of concentration needed would necessarily be impossible to attain.
Andalite_Shorm, that had not occurred to me, but I would assume that it would be possible to realize who was projecting the image, yes. Thought-speak can be identified, after all, despite the fact that it's not really a 'voice' that would necessarily sound like the spoken voice of whoever was projecting it. If projected thoughts can be identified, it's reasonable to assume that projected images could, too. Interesting point.
Aquilai, you bring up some very interesting ideas. Not only is it likely that some individuals might be better at projecting their thoughts than others, there's actually even some evidence for that being the case in canon. Gafinilan and Mertil, whose thought-speak could be heard around the world? Heh, Mertil could have had some fun with Gafinilan, and of course he would also be immune to any retribution on Gafinilan's part (maybe that's what
really happened to Mertil's tail
).
Anyway, I also like your idea of programming a machine to project these images, basically a functional AMR. We know that this isn't how the actual AMR in the books worked, because Tobias never mentioned seeing a mental picture of an Andalite (or a hawk, if the machine could somehow also read minds and derive the natural form of a morpher), which is kind of a shame. But if a machine could be programmed with the images of morphers' true forms, it could work to target certain individuals without necessarily being aimed at them. Of course, it would have some drawbacks, like being unable to deal with a morpher whose true form is unknown or not programmed into the machine.
WAIT. Oh, man. [spoiler=Book #49]Remember in book #49, when the Yeerks were collecting blood samples and comparing them against animal DNA to look for humans who had morphs floating around in their blood? This proves that even morph-DNA can be detected from a blood sample. And the other blood samples, the ones 'drawn' in battle from their morphed forms, prove that they can tell which DNA is from the base form, as well. All the Yeerks would need would be a blood sample from a morpher, and BAM, they have a complete inventory of their natural form plus all of their morphs.[/spoiler]