would it be difficult/possible to use this as a smart card with the proper code?
Dev: https://github.com/aze00/gnuk/tree/efm32
Of course the GnuK site lists some STM32 boards designed specifically to run GnuK. Or if you don't care about open hardware you can buy a $2 STLink clone on aliexpress and flash GnuK on it.
IIRC GnuK uses the chopstx library which has already been ported (see https://github.com/im-tomu/chopstx). Not sure what else would need to be done?