Hello, I've found out that debian-installer-launcher is always preseeding the live- installer with some options. I've modified debian-installer-launcher to force- load apt-cdrom-setup udeb, which solved the problem. I've opened a bug report against debian-installer-launcher, which can be seen at [0]. If that patch can be included in the package, problem can be solved without modifying the d-i.
Best regards, Hakan [0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=843130
