Use data in a tablespace to populate another postgres database instance without actually inserting
A table belongs to one database. Period. You can share a tablespace among databases of the same DB-cluster, but you cannot share a table.

But you can use dblink for what you have in mind. This way you can query tables from a different database - even from a different DB-cluster on a different server. Simple SELECT queries on the same server are very fast. Try this search for code examples.

Or look at CREATE FOREIGN TABLE for a very similar functionality. For now, I prefer dblink, because it is mature and stable. The new SQL/MED features (Management of External Data) FOREIGN DATA WRAPPER and FOREIGN TABLE are still in the making ..