I remember a couple guys discussing this on speypages a few years back. I can't remember if they were using MOW tips, Rio replacement tips, or poly leaders, but it was working. I makes sense, they're about the right grain weight, about the right length, and tapered, why not use them? If you can find one that is close to what you want give it a try.
Lots of Spey guys experiment and slice and dice lines. I've built short single hand Skagits out of old Spey and heavy SH lines I had lying around. These experiments all worked, were fun to cast, and I learned a lot along the way about head design. In the end I realized my experiments would never cast as nicely as a production line designed to do exactly what I was trying to do. I ended up buying an OPST line, and it was a much better casting line than any of my homemade heads.