So far, using RSA-PSS, it is caller's responsibility to prepare the input data like:
(data (flag pss) (hash <algo> <hashed-value>) (random-override <salt>))
with <HASHED-VALUE> (it is the caller who does hashing, beforehand).
It is good to have "prehash" function (in RFC 8032 (EdDSA) terminology) which hashes input internally when pk_sign-ed. The data format to use this functionality will be:
(data (flags pss) (hash-algo <algo>) (value <value>))