It's generally not a good to encourage manual entry -- those identifiers help aid grouping of duplicates on the ORCID record! That said, you could go with language such as "you don't need to include an identifier for the work, such as a DOI or ISBN, but we strongly encourage that you include one. ORCID groups works based on identifiers, and you can reduce duplicates by including an identifier". (Be sure to describe the relationship option as well!)
Regarding your institution not having identifiers: Definitely reach out to your regional ORCID engagement team member at sup...@orcid.org (or consortium lead), as other institutions were in a similar situation. If the work lacks, sat, a DOI, you can use a unique identifier from your system for the work, and set it as a "source-work-id". You could even have *all* works have their corresponding "source-work-id" added -- along with all other identifiers you may know for the work, e.g. PMID, DOI, ISSN.
Warm regards,
Alainna
California Digital Library
http://uc-oa.info