I have the 18.5" WSM and not a 22.5", but I bet this idea might work.
The 18.5" WSM can use a charcoal grate from the 18.5" kettle grill which is 13.5" in diameter. This grate fits perfectly inside the fire ring on top of the original grate. Here is a photo of the 18.5" WSM grate with the 13.5" extra grate set at 90 degrees like you are looking for.
The 22" WSM has a 17" diameter fire ring and the 18" WSM charcoal grate is 15 1/4" in diameter. So if you get a grate from an 18.5" WSM, it should fit and leave about 3/4 inch around the edge (the same way the 13.5" grate fits inside the 18" WSM like in the photo above.
63013 should be the part number for the charcoal grate for the 18.5" WSM
As to why do this.....
In the original 18.5" WSM grates (pre-2009 when they redesigned the unit), the charcoal grate had two large gaps at the outer edges and coals would fall down. The crossed grates will keep smaller pieces of hot coals on the grate in a mass which is where you want it. Here is a photo of a grate with expanded metal. You can also see the open ends in this photo.