Thanks to feedback by Mario 'BitKoenig' Holbe (his PM with Message-ID: 
20080512102019.GA19987@darkside.22.kls.lan), quoting some parts of his mails:

-APPEND apm=power-off lang=us vga=791 quiet initrd=initrd.gz boot=live nomce
-# DEFAULT menu.c32
+APPEND apm=power-off lang=us vga=791 quiet initrd=/boot/grml/initrd.gz 
boot=live nomce
+# DEFAULT /boot/addons/menu.c32

Evtl. koennte man auch syslinux.cfg aus dem / wegkriegen...

| Starting with version 3.35, the configuration file can also be in
| either the /boot/syslinux or /syslinux directories (searched in that

Und evtl. auch ldlinux.sys...

| This will alter the boot sector on the disk and copy a file named
| LDLINUX.SYS into its root directory (or a subdirectory, if the -d
| option is specified.)

Und gegen Redundanzen zischen isolinux.cfg und syslinux.cfg koennte das
hier helfen...

| INCLUDE filename
|         Inserts the contents of another file at this point in the
|         configuration file. Files can currently be nested up to 16
|         levels deep, but it is not guaranteed that more than 8 levels
|         will be supported in the future.

Und zum Thema multi-ISO - aber das kennst Du sicher schon...

| LABEL label
| ...
|   CONFIG image                  - New configuration file
| ...
|         CONFIG means restart the boot loader using a different
|         configuration file.