The only thing you can do is ignore her while she is meowing. Give her lots of attention when she is quiet. Do NOT let her back out on the harness right now, or she will learn that meowing enough gets her what she wants. My boyfriend's Siamese does this non-stop because he always gives in.
1) is she spayed? she may want to get out to mate if not
2) is she a siamese? or part? they are very vocal cats and you really can't do anything about it.
I can't imagine getting annoyed with my cats meowing, of my 7 cats, one has a very deep throaty meow and he is so loud but I think it is sooo adorable!