Tails OS - Modifying boot options

Chainload the ISO's bootloader using syslinux, then you can add all the extra parameters you need. Use the actual contents of the syslinux directory in the ISO as a base.

There are many examples online. Check this one.

There's actually another way to do it by remounting the live disk as writable

mount -o remount -w /lib/live/mount/medium

and then editing the live config file corresponding to your architecture (either live.cfg, live486.cfg or liveamd64.cfg) under /lib/live/mount/medium/syslinux
more info here

This path is incorrect for Tails. Actually I believe that the actual config files are here

/lib/live/mount/medium/EFI/BOOT which I have changed live.cfg and live64.cfg and this works. Re-boot and inspect the config file and see your changes.