I tried it on a headshaker I had a couple of years ago - didn't make any difference to him I'm afraid, but I think it's trial and error with a lot of these things...
Haven't used Shaker Guard, but my sister did use Global Herbs Scratch and Zephyr with excellent results if that is any help.
Plus any calming remedy with high levels of Magnesium may help in some cases. Certainly my sister's boy is on Steady Up by Feedmark and this has a very positive effect on his head shaking.
Not sure i've seen on of these, I use a nose net on my horse and it stopped him straight away. I tend to use it all year now, but if I don't in the summer he goes back to head shaking immediately.