Hmmm...the picture (on the page you linked) says the bracers are leather....and then the text says there was a bracer of sandstone.
![Huh ???](http://www.primitivearcher.com/smf/Smileys/default/huh.gif)
I've never made a bracer of sandstone, but I have worked with sandstone (a little) and have made a few arrow tools with it (arrow polishers and nock shapers). I can tell you that sandstone does not stand up to shock very well and would probably break if used for a bracer unless it was small, thick, or both. Drilling the holes in the sandstone would be easy but the holes would be a weak point and I question the effectiveness of attaching the bracer to the arm in this way. It would work better as an "insert" to stiffen a leather bracer, for example.
Anyway, as always, just try it and see how it works.
![Grin ;D](http://www.primitivearcher.com/smf/Smileys/default/grin.gif)