dtic, operating the free-hanging platen is very easy actually, it's as if the book pages somehow just guide it into right position. Hard to explain, must be tried out to understand how it feels like. Other users of this scanner found it easy, too. This is because you don't have to actually aim it, because it slides on pages. You can even use it to "flatten" the book a bit if the binding is tight and pages are slightly curved because of this.
jck57, that's actually what I originally planned
But time ran a bit short, and I didn't have time to build the mechanism until the scanner had to be ready to be used, and what was supposed to be a "one weekend" project turned out to last seven months, the scanner hasn't still returned home... Foot-operable platen is on "wishlist" for the next, smaller version of the scanner.