I took a look around last evening, and I added more info in this changeset: changeset/48443265. The washrooms are in a fenced off area, and if the gates are closed then they are inaccessible. I assume they are locked when the gates are locked.

The Change Rooms next to Hellard Field might have bathrooms on the sides facing the bleachers, but the gates were closed and I couldn't see from the road. The change room doors can be seen in this photo: https://www.flickr.com/photos/jpbadger/33650458384/in/album-72157683395996716/.

Some photos I took for more source information: https://www.flickr.com/photos/jpbadger/albums/72157683395996716