openssl and external card reader support in TLS

[Tobias.Wolf]

I need to implement support for the external authentication of a card reader 
within a TLS handshake. We did this already with PKCS11 using the C_Sign 
function and it is working fine.
Now I need to implement the same functionality in another use case with openssl 
for TLS handshake.
My Question is there a callback I can use or do I need to implement my own 
ENGINE? I want to avoid implementing the ENGINE interface, because that is a 
lot of work and maybe over engineered for this scenario, right?