It appears this can be done. I’m unable to determine the version of GCC that it was added, but it was sometime before June 2010.
Here’s an example:
#pragma GCC diagnostic error "-Wuninitialized"
foo(a); /* error is given for this one */
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wuninitialized"
foo(b); /* no diagnostic for this one */
#pragma GCC diagnostic pop
foo(c); /* error is given for this one */
#pragma GCC diagnostic pop
foo(d); /* depends on command line options */