Here's where my concerns would be:
1. Impact on handling and stability.
Seat stay is a load bearing component both vertically, and to a lesser extent torsionally under pedaling load. How does the asymmetrical construction material affect the rear triangle? As you have a full tube of one component on one side, and a cut tube with a different material on the other, will this cause any twisting or deflection of the rear wheel under loads? Additionally, if this is an end user installation, how do you ensure that the end user makes their cuts the proper length, changing the length of the stay and thus rear wheel alignment?
2. Frame warranty.
This obviously voids any warranty on the frame. Will you accept liability for any other frame issues that would have otherwise been covered by the manufacturer?
3. Questions I'd ask if I were an investor... What is the market?
Given 1 and 2, what is the marketability of the product? I can see that it does serve a purpose, and I would maybe entertain doing it on an old beater fixie, but will it be priced low enough to be a viable product? I'm not going to spend $100 to perform this upgrade on a frame thats worth < $400, and I'm not going to cut an inch or two of seat stay out of one that's more than $700. So can you produce it and make money off of it selling it for $25?
Additionally, will shops actually stock it? I think you might have a problem getting it in a bike shop with them knowing they (or the customer) will have to destroy the frame to install it.
I don't mean to be negative. I think it's a good idea and I do think that there is a market for it in some areas, but I think it's extremely limited.