Steve,
You have such a wealth of information!  Is that the Griswold #18 you are talking about??  It does look like a cookie sheet, but is marked as a "Family Grill."  It looks nice and wide.......Joe will have to fry his bacon on that instead of baking cookies.  I wouldn't mind having a griddle that was a bit wider than my Wagner......isn't quite wide enough for the bacon, and I have to "smoosh" it up a bit when I put it on.  
I'm not sure a cast iron cookie sheet would be practical....I think it would be very hard to get out of the oven....considering the size and weight.   I have a bit of trouble with some of my DO's, and I know my Wagner griddle weighs a ton.  I would hate to pull that out of the oven!  I think Steve may be right....and I never even thought about it being a griddle.  See how much you learn on this forum??