There is a "postgres" option -o which can redirect the server output to a
file.  The postmaster output will still go to stderr.  Does anyone recall
how this was supposed to be useful?
Interesting code (from elog):
    /* NOTE: debug file is typically pointed at stderr */
    write(Debugfile, msg_buf, len);
    ...
    /*
     * fflush here is just to improve the odds that we get to see
     * the error message, in case things are so hosed that
     * proc_exit crashes. [...]
     */
    fflush(stdout);
    fflush(stderr);
Fat chance.  ;-)
-- 
Peter Eisentraut   peter_e(at)gmx(dot)net   http://funkturm.homeip.net/~peter