Please consider adding Yubikey support as an alternative to smart card. Potential benefits:
Yubikey readily available in many markets (so is smart card)
Supports more "out of the box" experience: doesn't require additional shield or PCB. Just power DIY from E5V or STLK connections, and plugs into micro-USB (will require user to have a USB to micro-USB adaptor cable, probably).
Please consider adding Yubikey support as an alternative to smart card. Potential benefits: