That's quite a massive topic - indeed you are right: the "frontend" API for the OpenPGP Card has not been started yet, but this is by far not the challenging part, especially as this is solved by projects like "gnuk" already and will need adaptations, but no from scratch implementation. The challenging part is the backend and the Trussed-pipeline. With backend I mean the translation of the OpenPGP Card frontend api-calls into the SE050 secure element world, which of our best knowledge has not been done before. This is where most of the effort is currently invested, not to mention the architectural work needed within Trussed to support crypto-hardware-backends. A lot of groundwork work has already been done like a Trussed-firmware-simulator to ease development, the actual OpenPGP Card frontend is really just the tip of the iceberg...