I think you're on the right track with using larger cuts of meat that can be cooked first, then assembled for serving. That should save some time. What you can assemble first and cook on site will offset some of your onsite prep time so that part of it is ready straight out of the cooker, while the rest is cooked, then assembled. Finding the right balance of the two will be helpful...just thinking along the lines of multitasking here.
For appetizer ideas, I'm drawing somewhat of a blank, as I don't do them very much. OK, wait...
For onsite prep after cooking:
Pulled Pork or Chicken Quesadilla, with grated pepper-jack for a little kick. Some could be with a mix of cheddar for a heavier flavor. Also, for the heat-seekers, add some chopped Japs, seeds and all (prepped off-site), to a couple of them...just make sure they know what it is. Brush on bit of BBQ sauce before dropping everything on to finish it up. These will be a huge time saver, I think, as you could make a few dozen from one assembly, depending on the size you slice them to.
Eric