bootoptions are not passed to Ubuntu ISOs

[probonopd]

Ubuntu contains a /boot/grub/loopback.cfg which references ${iso_path} but not ${kernelopts} so setting CUSTOM_BOOTOPTIONS in /etc/default/grml-rescueboot will have no effect on Ubuntu distributions.

Probably this should be submitted to the Ubuntu developers working on loopback.cfg.

Another way would be to have /etc/grub.d/42_iso copy the contents of loopback.cfg (rather than including it) and have it add ${kernelopts} on the fly. That way, existing Ubuntu ISOs would not need to be changed.