Although the PKCS#11 representation is callable from C, there is likely a need for a simpler C API and ABI to access the trust information.
At the present time no C caller for such an API exists. Most crypto libraries already use PKCS#11 to access keys and certificates, and thus they are not the consumer for this C API. It is also undesirable to design an API in a vacuum. Therefore if you are such an interested caller, please contact p11-glue@lists.freedesktop.org for discussion.