currently all conga config parameters are placed 1:1 in the generated target files (or perhaps escaped with escaping rules required for the target files).
there are use cases where we need to process the value before placing it in the target file, e.g.:
encrypt a publisher password using AEM Granite Crypto Support for storage in JCR
build a password hash for a oak user based on his password
build a UUID for a oak user based on it's principal (if there is really such a dependency)
the processing could be executed in the handlebars templates using a specific handlebars expression.
it should be possible to pass in optional parameters to the value processor, e.g. the hasing alogrithm to be used.