I don't know how much expertise you have, so forgive me if this is too basic to you, I'm not trying to be insulting. :-)
The problem is that it is trying to launch the installer on the "PC" display adapter and you want it to use the serial port.
The installer is launched by init and the command line is in inittab in the initial ramfs. Here is a step by step set of instructions.
- Mount the USB stick, you've already done that, so we'll call whatever directory you mount it on /usbstick.
- The installation ramfs is in a compressed fs image in the file /boot/isolinux/instroot.gz. Decompress it to someplace like /var/tmp.
gzip -cd /usbstick/boot/isolinux/instroot.gz >/var/tmp/instroot.img- Mount the instroot fs, I'll use /mnt in this example
mount -o loop /var/tmp/instroot.img /mnt- Edit the inittab appropriately, I would remove the lines for tty3 and tty5 and change /dev/tty1 to /dev/ttyS0, change /dev/tty2 to /tmp/installer.log.
- Unmount the image
umount /mnt- Compress it and copy it back to the USB stick
gzip -9c /var/tmp/instroot.img > /usbstick/boot/isolinux/instroot.gz