Probably an Arduino based board, I don't think Pi's hardware is open (yet?) and trying to source that in bulk would only be a pain in the ass instead of actually saving money. Will check out some other microcontrollers, should be possible to find one that doesn't struggle with signing transactions and is "open". Something like
this.
Yes, please. I'd be one of your first customers. I am also happy to contribute code and testing, too. I've an arduino already so if you get a code repo setup I can help test. I've got an NFC board that I'd be happy to add support for too:)