Please refer to the answers that have already been added to this question under the Nemeth section of the Forum.
I, too, am very uncomfortable making a suggestion contrary to what the Guidelines and Standards propose. Having said that, they [u]are[/u] guidelines. Rather than using the shape indicator, as you suggested, I think you would be better to use an alphabetic key. As stated on the Nemeth section of the Forum, students are usually give classroom manipulatives (Base 10 blocks) when working with this concept. One of the textbooks that I transcribed used the terms cubes, flats, rods, and ones, but you should use the same terminology as is used in the surrounding text of your book. Obviously, since the contraction for one is only 2 cells, it would not have to be keyed.
At times, these are depicted with a group of 10's being moved to the ones to show the carrying of numbers for subtraction, or vice versa for addition. In such a case you would still need a tactile graphic to circle the symbols used and an arrow showing where they are moving. Alternatively, if you are going to have to create a tactile anyway (to show movement), you could make a simplified shape (a large square for 100 without showing the individual squares, a long rectangle for the 10's without showing the individual squares, and a small square for the ones). These would also have to be explained in a transcriber's note. In books where there are several pages of Base 10 blocks the first couple at least should be done as tactiles according to the Guidelines and Standards, and then your modified method explained and used for the rest.