cloud-init seems to have quite a learning curve and since I am working with an existing droplet, I took what seemed like a much easier path.
I added to my /etc/rc.conf the lines to rename the interfece, but then also added the lines that DigitalOcean is adding, edited with the new if name so that I get the correct IP address etc. I ended up with the following lines added.
ifconfigwan="inet XX.XX.XX.XX netmask 255.255.255.0"
ifconfigwan_alias0="inet XX.XX.X.X netmask 255.255.0.0"
DigitalOcean deletes are re-appends lines on each reboot, which I just ignore. Although they are now trying to assign an IP no a Interface (vtnet0) which does not exist, they seem to cause no problems or negative effects. Since the IP is static on the Droplet, it should be no problem.
If I do discover any subtle issues, I will modify their script to append them as commented lines.
The script that does the deleting/appending is
This script could be vastly better if they just edited their lines in place, maybe locating them by having the comment on the end of the line instead of the line above. If they did an in-place edit, I could simply have my interface rename at the end of the script which should be much cleaner.