Me? I'd think twice about the plate thing. 2 reasons. One not sure what that might do to the components in the fire box. Remember it was engineered for a level of heat loss through those grates. You could turn things in to some serious over heat conditions. Second IMO it's a FAR better grill than griddle. If anything I'd maybe do just half of it as griddle. BTW the trouble area on these is the front portion of the fire box where the burner tubes go through to meet the control valves. They burn/rust through there
An interesting thought, why not make it so the left or right half is a griddle and the opposite half is a grill. Might be less of a problem with to much heat and will maybe be more practical.
THere's proabably a Weber made griddle insert specifically designed for Summit's size that will slip right in there without the need for fabrication. Weber calls their griddles "planchas" which I think is what they are called in Europe.
I'd be more concerned with how you manage the grease. With grates, liquids are dripping more or less evenly directly off the food. With the griddles, you could get larger volumes all running off in one spot, causing very big flare ups (a good reason not to ditch the flavorizer bars) and/or overpowering the flavorizers' ability to vaporize it and giving you a gush of hot grease overfilling your drip pan.
Does that make sense? Could your buddy route a groove around or at the fronts of the plate to give you some control over where the grease flows?
Or it might not be a problem at all, just a thought.