I always wondered why it couldn't be approached this way, and the response I usually got from mfg.'s was that there isn't enough demand to justify a full production run. But if you look around the market there are lots of options out there that seem to sell well for this platform, so good on you for tackling it. Looks like a nice, well engineered, thoughtful design.
Assuming you calculated for proper volume, my only concern would be potential clearance issues on abnormally low bikes (IRK). How far down does this go past the bottom of the frame rails?