I read someplace that garlic powder added to their feed also helps with the smell. I haven't tried it. It sounds interesting because it might also boost their immune system. Mint has a very strong taste and my ducks only nibble on it occasionally. I think adding any herbs that you have trimmed or cut back and let dry out in the sun would be great bedding additions. Lavender! Far better than throwing it away. It would act like a pen potpurri. They wouldn't eat it though.
If the stinky area is small enough, digging and working the old hay shaving into the soil really helps. The soil will be enriched, less compact with better drainage and airflow. Than spread a new layer of hay, sweet pdz and DE. I do this even in areas I have pea gravel. The gravel is still there helping with drainage under the surface but it does mean I have to with top with more gravel or hay.
The only time I'm really glad to be living in a dry climate is when I'm dealing with the whole duck mud problem. But, oh what I would give for a little stream or a natural pond! Good luck