Yes, Walt used very short hackle tips as cheeks, not sides, on some of his spey flies, not all of them as I previously mentioned. The reason I said cheeks, not sides, is because Walt tied them very short in the manner of Indian Crow or Chatterer feather cheeks that are put on over jungle cock on married wing classics. In fact, he referred to them as cheeks himself.
And he tied them much shorter than you did. His were maybe 1/8th of the wing long. However, others have used very short, dyed hackle tips, not fibers stripped from a hackle, as short cheeks instead of IC or Chatterer for many years before Walt used them on some of his spey flies. In other words, Walt wasn't the first one who used them. Bill Hunter, the fellow responsible for the HMH tying vise, used them and said he learned it from Ira Gruber.
In my opinion, something that is a "signature" of a fly tyer is something he or she does or did on every fly, such as Carrie Stevens and her use of a band of red thread in the head of each of her featherwing streamers. Since Walt did not use these short hackle tip cheeks on each of his spey flies, I don't consider it to be a "signature" or identifying characteristic of his flies of fly tying.
What I'm getting at is that Walt tied many flies, including spey flies, that did not have toppings over the wing or short cheeks of hackle tips. In fact, he also used very small and short dyed Lady Amhearst tippets for short cheeks. For example, his ROYAL SPEY. Does that mean the use of dyed LA tippets tied as short cheeks over the wing sides is also a "signature" of Walts?