not that I'm aware of. I installed Asuswrt-Merlin only a few days ago specifically for custom DDNS feature and that's all I've played with
EDIT: isn't syslog in /tmp/?
EDIT2:
grep "ddns" /tmp/syslog.log
Feb 27 09:53:35 ddns: Custom ddns update failed
I'm using the Merlin Wiki script for DDNS for Cloudflare. It works from terminal in Merlin but how do I know it's working autonomously? The built-in DDNS services have "Registration Successful" message in the DDNS GUI webpage but I don't see any status for Custom script even though its posting...