OK, I'll take a shot. The guage shown combines a piston guage with a dial indicator. The idea of the bottom "feeler" is that it moves up as the piston moves up at the top of the stroke, but is directly connected to the dial indicator. To me. it looks like it could work, but I'm hesitatant about the range that it can cover, as well as the ability to fit the threads on the spark plug holes in the heads.
You can get ordinary piston guages, such as these from Eastwoods:
You can add a dial guage with a magnetic mount (even easier if you have steel valve covers), to be extra accurate. I haven't seen an all in one like the one originally shown, but its a neat idea. Using a speparate dial indicator has the advantage that your dial indicator is available for other tasks.
As far as needing one to set valve lash accurately, its not really needed. You need the cam to be on its base circle when you set lash, but each valve is on its base circle for quite a bit of duration. If you put #1 at TDC at the top of the compression stroke, both the intake and exhaust valves should be fully closed (and have been fully closed for a bit, and will remain fully closed for a bit) so that you can set valve lash. As long as you know its TDC for the compression stroke, it is as accurate as you're going to get. Plus or minus 15 degrees should still be on the base circle. If you're at TDC betwen the exhaust and intake strokes, you won't be on the base circle.
If you are having trouble seeing the mark on the harmonic balancer, you can rub the marks with a Markal pen, then wipe the area down, which leaves the crayon in the markings on the dampener to enhance visibility. Another option is to buy a timing tape to stick to the dampener, which gives you markings all the way around the dampener (my preferred method, since I use the rotate 90 degrees method of setting lash.) You can get the timing tapes at your local speed shop, or order them through Summit:
Once you've established TDC at the top of the compression stroke, set the lash on both intake and exhaust for number 1, then each 90 degrees of rotation, set the next pair in the same order as the firing order. There are other methods that work to set up the valve being on the base circle of the cam, its just whatever you prefer.
The accuracy as far as TDC that you are proposing to use makes sense for degreeing a cam, but isn't necessary for setting valve lash.