query SQL Server from PostgreSQL
In addition to the standard but immature SQL/MED and foreign data wrappers mentioned by @vyegorov, the traditional solution to this problem has been DBI-Link, a set of foreign data mapping functions that use Perl DBI via plperl.
There is a SQL/MED extension that describes how database can access external data.
PostgreSQL supports this part of the standard via the Foreign Data Wrappers. The linked page describes all (well, almost all I suppose) FDWs that are available now. I think that your finding on odbc_fdw are correct, but I admit, that it is not always straightforward to make FDWs working, especially on Windows.
You can take a look at Multicorn if you're familiar with Python.