What is the cemetery exposure like? If its's shady you could try the hostas. If it is full blazing sun stick with sun loving drought tolerant perennials. Other than that you came up with a great idea. just use you imagination.

I use a terra cotta pot, bottom 2 inches of styrofoam, peanuts or chunks of broken packing. Terra cotta keeps plants wetter than just plastic, with styrofoam giving good drainage so no roots are soaking in water. I can only water every couple weeks if no rain. This pot has been very good for a number of years. I pick it up and store at home for winter, freezing would crack and break it.

I use annuals since our cemetary allows pots all summer. Sweep potato vine was nice along with new plants I had never seen. I just went shopping, tried stuff to see what would work. I don't do perennials because they only bloom once while annuals keep the show going all summer. You might try the annuals, enjoy the pot at home too.