libcmis should allow clients to connect to CMIS servers that require client authentication with certificates. libcurl, the library used by libcmis, already exposes this functionality via the following options: CURLOPT_SSLCERT, CURLOPT_SSLCERTTYPE, CURLOPT_SSLKEY, and CURLOPT_SSLKEYPASSWD. The libcmis SessionFactory and Session would likely require modification in order to implement this feature.
Do you have a minimal test server we could use to test this while implementing it?