Dynamic binding in plpgsql function

Hi,

I would like to write a generic plpgsql function with a text parameter being a callback function name so that my
generalfunction can call this callback function. e.g.: 

CREATE OR REPLACE FUNCTION ST_MyCallbackFunction(y int)
    RETURNS int AS $$
    DECLARE
    BEGIN
        RETURN someCalculationBasedOnY;
    END;
    $$ LANGUAGE 'plpgsql';

CREATE OR REPLACE FUNCTION ST_MyGeneralFunction(callback text)
    RETURNS SETOF geomval AS $$
    DECLARE
        x integer;
        y integer;
    BEGIN
        y := somecalculation;
        x := 'callback'(y);  --This is what I need
        RETURN x;
    END;
    $$ LANGUAGE 'plpgsql';

I don't want to do an EXECUTE statement since I have no table to put after the FROM clause. I want to assign the
resultingvalue directly to a variable like in my example. 

Can I/How can I achieve this?

Thanks,

Pierre