>I've tried to partition the disk (a 105 MB Syquest in case it is relevant)
>with a mix of several applications (Silverlining, pdisk under Linux, disklabel
>under netbsd) and made file systems with newfs.

pdisk is not needed (has no effect) because NetBSD/macppc currently
don't use Apple partition map.

>My problem is how to make the disk bootable. I've tried using installboot
>and installing the boot code present in arch/macppc/stand, but without
>success -- Open Firmware insists that the partition is not bootable.

# /usr/mdec/installboot /mnt/boot /usr/mdec/bootxx /dev/rsdXX  should work.

installboot writes boot block and *fake* Apple partition map entry for
Open Firmware.