Does anyone make a cleat, or an adapter that would accommodate a cleat, that has a slot for use with old style caged pedals that use toeclips & straps, that fits 2-bolt SPD style cycling shoes?
I did find these
And Yellow Jersey is exactly the kind of shop you'd expect to find that stuff. The one in the image is for Road shoes, but I think could easily be adapted to SPD shoes with some work with a dremel tool.
They say they will make a custom cleat for just about any shoe. You do realize that because SPD shoes have the cleats embedded in the outsole, you'll need to at least grind a slot in the tread of the shoe.
It occurs to me that just grinding the slot in the tread of the shoe and then grinding off the tabs on a standard SPD cleat would get you pretty close to what is required.