A major issue that the use of Open Access content such as unpaywall can have is the version and making people aware of the version they are seen/given them the choice of version (submitted/author accepted/version of record)
For institutions who have Unpaywall collection available and/or their institutional repository data being added in, use the metadata in the background of the record to identify the version that will be available from the link (signpost would also ideally be applied in the Brief record view and/or full record) and when Open Access facet is selected, it would automatically open an additional Facet for
- submitted
- accepted
- published

to allow users to limit their results by version available within the unpaywall collection