I would suggest patching the original inittab and autoconfig.functions to only
enable the serial console on the device that was actually given on the command
line. That way, there would only be a getty on ttyS0 if "serial" were given on
the grml command line, and on ttySx if "serial console=ttySx,..." were given.

Actually, the getty on the "other" serial port may cause unintended behavior

The (only) line in inittab mentioning the serial console would be:

#grmlserial#T0:23:respawn:/sbin/getty -L #grmlserialdevice# -l /usr/bin/zsh-
login 115200,57600,38400,19200,9600,4800,2400,1200 vt100

and the autoconfig function would do something along the lines of

sed -i '/^#grmlserial#/{s/#grmlserialdevice#/$TTYDEVICE/;s/^#grmlserial#//;}'
/etc/inittab

Please indicate if such a patch would be acceptable to you.

Greetings
Marc