I had this problem and i sent a ticket to ESRI.
I had two layers, one a tiledservice (MrSID imagery) and a dynamicservice of polygons.
The inside of the polygons were empty but when adding the polygon over the tiled service, the transparency and the antialiasing weren't working at all. But when we set the image format to png 32 of the dynamicservice with your trick, it all work fine.
But, if you check the documentation in the API, it says that you need to use the function setImageFormat or the pass the parameter imageFormat in the creation of you TiledService, but it won't work. But with your trick, it works very well.