Sounds like a great plan! Mario's advise is a good one. If you can make the shield 'daisychainable' it would be awesome!
Hey, Thanks for the input. I wasn't planning on making these or selling them, it's just a project for me to update my PCB making skills. But once I've got the board working and debugged I'll post the cad files and a write up on my blog.
Of course, if you want the files now, just shout and I'll chuck them and some code into github.
L