You could plan two mains, one with meats and no worries about seeds, etc.
The second one could be vegetarian using absolutely no seeds (roma tomatoes, meaty and are easy to deseed) for those with diverticulosis type issues.
Whether they are casseroles or kebabs for the BBQ, it can work for either. If the chosen vegies are too soft for a kebab there are always foil packs.
Sides should be easy enough to separate. Potato Salad, Egg Salad (have to be careful there - alot of add-ins will have seeds), Three Bean Salad would be great and loved by all!
Are you sitting to eat? or buffet style? In either case, labels should be easy. I would just specify "meat" or "seeds". You supply and present, you don't have to babysit.
How big is the gathering? Big enough to need more than the two mains? If so then perhaps the guests could contribute a dish to reflect their needs.
Again, two ways to go there - guests bring the sides and let you concentrate on the mains.
or guests bring the mains and let you concentrate on the sides and little special extras.
Or you could be entirely creative and make it entirely vegetarian - Larry would have some excellant recipes for you
and they probably wouldn't even know they were vegetarian! For those with intestinal issues, it might be a little harder but I should think doable!
Hope this gives you a few ideas to start with
oh, and btw
Welcome to DC!