I use a USB token to access many work systems. This is supported by OpenSSH as a build-time option. It provides an interface to OpenSC, the open-source SmartCard support package. gnome-keyring lacks support for SmartCards which use the same software and standards as the USB tokens. (They are effectively a SmartCard and reader in one package.)

All code is available in OpenSSH-portable, but you may need to re-write if you need it under GPL. (Of course, OpenSSH is already in the system with a BSD license, so you may want ot just use the code and leave the BSD license. IANAL.

When it is included, the code to get hte passsphrase will need the ability to request a PIN for the SMartCard instead of the pass phrase. (The dialog could either provide a radio button to select or have two entry lines if a SmartCard is present.)

OpenSSH-portable needs to be configured with --with-opensc to work with SmartCards and is dependent on OpenSC (and, probably, OpenCT).