How to import an existing schema and tablespace into a postgres database, from a filesystem-level backup
Say I have an existing database, and I want to add to it another
  schema with content. I saved that content previously through a
  filesystem-level backup of the tablespace that served the schema's
  tables and index


Unfortunately, an isolated backup of a tablespace cannot be used for any purpose whatsoever.

I think what you're trying to do is based on the  premise that a tablespace could be more or less moved around, be possibly plugged to a different database or the same database at a different point in time, that kind of thing. But none of this is possible. A tablespace holds live metadata in rows and elsewhere that become wrong the moment you isolate it from the rest of the instance.

The results of DDL statements go into the pg_catalog schema of each database. pg_catalog also contains references to objects that are shared across all databases and not stored in any particular database, such as pg_authid or pg_database.

Looking at Have postgres&#39; pg_dump export an index I can see where your idea is coming from. However I don't see how @mustaccio answer has any practical use. If you have a FS-level image of a table plus its indexes, they having matching TIDs, and then what? There's no practical use to these files outside of the rest of the cluster.