How to set default mount options? I'm trying to figure out where the default mount options are set for user-mounted filesystems (e.g., disk images, USB thumbdrives, etc.). I'm working with OS X 10.11 ("El Capitan").

The options I specify for known disk partitions in /etc/fstab are respected when they're mounted automatically at boot time — or even manually via Disk Utility — but I can't seem to set the default mount options for any "unknown" filesystem that gets automounted in /Volumes.

I've tried changing AUTOMOUNTD_MNTOPTS in /etc/autofs.conf and adding options with -o to the ProgramArguments array of /System/Library/LaunchDaemons/com.apple.automountd.plist (which required disabling SIP), but neither change seems to have any effect.

It would seem that some other set of defaults is being used... but where are those defaults defined?
If I understand your question, you should have a look into /etc/auto_master

This article explains very well how auto-mount works
https://useyourloaf.com/blog/using-the-mac-os-x-automounter/