I just double-checked this again on a usb stick I wiped clean with fdisk and formatted ext2.
Using syslinux-4.05 and the core-4.2.iso, I see "-i" is now required (info file updated accordingly), but otherwise things work as per the info file example, i.e. : $ sudo mkdir -p /mnt/sda1/boot/extlinux
$ sudo extlinux -i /mnt/sda1/boot/extlinux
$ sudo cp /path-to-file/isolinux.cfg /mnt/sda1/boot/extlinux/extlinux.conf
..and then: $ sudo cp /path-to-file/core.gz /mnt/sda1/boot
$ sudo cp /path-to-file/vmlinuz /mnt/sda1/boot
..and things work without even needing to edit extlinux.conf