This is an update to this story.
In the end, I'm going to get paid for my maternity leave. It was an honest mistake - I finally found out that my HR rep called our corporate office to find out if I'd be covered under the new insurance company, and her contact there made a mistake and said I would be covered. I had to do some prodding to get them to fix it, but I'm finally getting paid next week. Seven weeks' pay in one check, woot!