Queries w/ "computed" table names? (eval in Pg?)

Hi. Suppose I have a database that contains a "meta table" that holds the
names of other the tables in the database, keyed by human-readable but
longish strings. I would like to write queries that first "compute" the
names of some tables (i.e. by looking them up in "meta table"), and after
that they execute subqueries using these computed table names. The
following invalid SQL illustrates the kind of maneuver I'd like to do:
SELECT x, y, z
FROM [ SELECT table_name FROM meta_table
WHERE human_readable_key =
'some veeeery long and unwieldy string' ];

The stuff in [ brackets ] is not meant to be valid SQL, but rather to
suggest that the name of the table for the "outer" query corresponds to the
string returned by the "inner" (bracketed) query.

Some programming languages allow the run-time evaluation of a string
representing some code in the language. One way to do what I'd like to do
is based on this idea: I would construct the source code for the desired
subquery as a string (including the name of the table obtained at run-time
from meta_table), and "somehow" evaluate this string. This "somehow" is
what I'm missing. Is there a way in PostgreSQL to evaluate a string as SQL?

TIA!

kynn