Slotted cleats for SPD shoes
I did find these 

http://www.yellowjersey.org/tocleat.html



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.