I have 18" diameter replacement grates for my Big Green Eggs that are 304, and they work great. These were part of a special production run, so I'm guessing the manufacturer had a jig to minimize warpage during fabrication. They look like a standard grill grate for a Weber kettle, only the rod is slightly heavier and there are two stiffeners under the main rods. When making my own BBQ accessories with 304 (in 1/8", 3/16" and 1/4" rod) warpage is an issue, so balancing the heat and making small TIG welds is mandatory.
For your application, there is one thiing you might look into..... custom grill grates. They are spendy up front. These guys come to mind as they are members of one of the BBQ Forums I hang out on. I do know they work in stainless, even though I did not see mention of it on their site.
http://www.huntingdoncocustoms.com/~thirdeye~