Hi
What is the price of this going to be?
Are you aware that you have competition already from the new MUXshield which also has lots of i/o pins (only 48 but when are you going to need 64?) and it is also stackable.
Why did you chose male instead of female headers? Is there a specific cable you can buy cheap for that size? I think that it would be good to have an option, so it can be used for quick prototyping or more permanent installation like yours would be best for at the moment.
I have a project though, more info about that later 
Mowcius