I have used all these bits and pieces to make pointers stuck to the frame and forks, and I have used the brake blocks, and they all work. But it can be fiddly to get a really pointy pointer that points exactly where I want it, at the bottom or top edge of the braking surface so that I can check for roundness as well as lateral truing. It would be nice to have something that can be let out by 1mm when I want that to happen.
I do have a truing stand and the reason i sometimes don't use it is because it can be easier and more convenient to keep the wheels in place and just do a quick job. So any "tool" that straps to the fame would need to be quicker to set up than just whipping the wheels out and putting them in the stand. In terms of time I'd be surprised if there were much in it, so I'm not sure this thing would be much use to me. Using the brake blocks or using the stand gives me all the options I need.