After starting this thread, I spoke with my friend who has the griddle I got to use. He pretty much does all his outdoor cooking on the griddle except steaks, which he cooks over a firepit with a camp grills. So after watching some youtube videos and getting more ideas on cooking on a griddle, we decided to go that route. The griddle is just so flexible and we learned of a variety of things we weren't thinking of. Also, we considered all of the things we typically cook on a grill, and except for smoking, they were all well suited to a griddle (steaks, burgers, hotdogs, peppers, onions, etc). Since we still have the Weber kettle, it's not as if we are locked into the griddle from here on out. What few tasks aren't well suited to the griddle will go onto the Weber.
As for cast iron pans on a regular grill, I've tried that and didn't like it. Again, the issue is with working space, of which there is none with a pan on a grill. We also looked at purpose-built griddles for grills, including some custom ones built to order, but the latter was $140+.
We settled on the 28" Blackstone. Cabelas has them for $240, but other vendors have them for as little as $154, so I went to Cabelas and asked about price matching. They price matched Home Depot, so I ordered mine on the spot, paying with my Cabelas points. Final price after tax was $164 and not a dime out of my pocket.
Chris