Well, I just figured out something that I think will satisfy me for now. I had noticed that the DHCP for the Docker subnet was assigning IPs alphabetically. Bitwarden was getting 172.17.0.2, Duplicati 172.17.0.3, Grafana 172.17.0.4, etc. I also noticed that you can drag Docker containers around on the Docker tab in Unraid. So, I dragged my Letsencrypt container to the top of the list, and then disabled and reenabled Docker in the Unraid settings. Sure enough, my Letsencrypt container now has 172.17.0.2.
Since I can control the sort order of the containers, I can effectively control the IP assignment. I'll put the dockers that need a static IP at the top of the list, and I'll ensure that any new containers I add go to the end of the list.