Quick hack for fetching the contents of a temp table

Hi,

Is there a quick way to fetch the contents of a temp table from within
postgres server code?

The temp table has a single column and there is a single value in the
column. Thus, I'm using the temp table as a variable to store a string.

begin;
create temp table mytemp(metastr char(1024)) on commit drop;

The metastr attribute in the temp table is populated by a function as shown
below.

insert into mytemp(metastr) (select somefunction1());

I need to use the return value of the function somefunction1() to modify a
query and execute it.

While it is easy to fetch the desired result using plpgsql functions(),
however modifying and rewriting the query using the plpgsql does not seem
straightforward. Therefore I have already written the query modification
function within the postgres server since I need to use the results from
pg_parse_query() method.

I stepped through the "select metastr from mytemp;" query only to find that
the process of fetching a tuple and the corresponding attribute is a bit
convoluted and I was wondering if something in the lines of this code is
possible: https://www.postgresql.org/docs/11/xfunc-c.html#id-1.8.3.13.10

From this code, it appears that the tuple has already been provided in t
through the statement:

HeapTupleHeader t = PG_GETARG_HEAPTUPLEHEADER(0);

and before that through the query:

SELECT name, c_overpaid(emp, 1500) AS overpaid
FROM emp
WHERE name = 'Bill' OR name = 'Sam';

What I'm looking for here is a method to fetch the tuple from mytemp
within the postgres server code and then extract the value from the
metastr attribute into a char[].
Since all statements above are within a transaction block, I think we
do not need to have read locks while accessing temp table.

Thanks,

-SB