Equivalent of MRG_MYISAM in databases other than MySQL? Does anyone know if other database have something equivalent to MRG_MYISAM (aka the MERGE table type/storage engine)? 

I know about fragmenting, but this is not quite the same AFAIK.  We're using MRG_MYISAM to avoid large amounts of duplicate data across customer specific databases, so MRG_MYISAM is perfect.  

That said, I'd like to know if there are equivalent things in other DBs, particularly other open source DBs.
From the description of the MERGE table type in the MySQL documentation at https://dev.mysql.com/doc/refman/5.0/en/merge-storage-engine.html it appears this is similar to creating a view that uses a UNION statement to connect multiple tables, either from a single database or multiple databases.

In Microsoft SQL Server (which admittedly is not open-source), you could do something like this:

CREATE VIEW MergedTablesView
AS
    SELECT Field1, Field2, Field3, ...
    FROM Database1.dbo.Table1
    UNION
    SELECT Field1, Field2, Field3, ...
    FROM Database1.dbo.Table2
    UNION
    SELECT Field1, Field2, Field3, ...
    FROM Database1.dbo.Table3

    ...


The UNION statement will remove duplicate rows from the result set, which seems to be your primary requirement.

I'm certain other database engines, including PostgreSQL, also implement the UNION keyword in a very similar manner.  Here is the PostgreSQL Manual on the UNION statement.