It turns out that the sync_exception isn’t anything very exciting: it can be caused by dereferencing an invalid address so it’s just a segfault, really. And the patch for hwkey_agent.c above omits calling HMAC_CTX_init, so the crash is not surprising. (I get confused writing code for different versions of OpenSSL.)
Trusty’s OpenSSL now seems to be working for me, apart from the problem described in another thread: Problem with OpenSSL: undefined reference to CRYPTO_gcm128_init
Edmund