My first thought was that the thing would be a mess. You would slip it in a sleeve to take care of that.
I saw you got no reply. In case you check back, I think you got no reply because many people here would not carry grills. I think.
If they do carry a grill, it might be too small to serve as a frame for a backpack. I can imagine some little +/- 7" inch square grill area with wire to bridge the remainder of the distance across the fire... This is the only thing I can imagine..
If the backpack has a frame, it is unlikely to be shaped for grilling. Some light packs that do have a frame just have a stay or two. It could, maybe, be used to skewer through something... many light packs just use plastic or foam as the frame.
Since these light, flexible materials are satisfactory for light packs, I think a grill against your back (even wrapped in some foam) might be uncomfortable or make it a little harder to move. If I remember right, even external frame packs have straps/bands that suspend the frame from contact with the person. The light frames in light packs do flex or give a bit, I think. Maybe. Unless the pack is stuffed tight like a brick.
Even many heavy packs don't have frames that would be suitable for grilling.. They still use the plastic sheet and toss in a aluminum stay or two.
I suppose the idea could be implemented. It might have value only for people that already do carry a grill. I doubt it will be attractive enough to others that do not currently carry a grill so they think 'hey, now I can carry the grill I would like to have'. I suspect sliding the grill in the sleeve might serve more as a unique place to store it than as a pack frame itself.