Gian-Paolo D Musumeci suggested using FWB Hard Disk Toolkit to remove the flag. Unfortunately, I don't own HDT. I mention it here for the next person who stumbles on this problem who might own HDT.

I read the quik source, and after looking everywhere for mac-part.h, found it packaged with quik. :v) The bootable flag is the bit masked by 0x08, in byte offset 0x05B from the beginning of a given partition. Partition descriptiors s are on 0x200-byte boundaries from the beginning of the disk, starting with the one at 0x200. I fixed this in my case by:

    1. booting off the installer floppy
    2. hitting ^Z to exit the installer
    3. mount /dev/sda6 /mnt (get to all my installed programs)
    4. chroot /mnt sh
    5. cd /
    6. dd if=/dev/sda of=/tmp/pmap size=512 count=10
    7. /usr/local/bin/beav /tmp/pmap    # edit partition map to remove flag
    8. dd if=/tmp/pmap of=/dev/sda size=512 count=10
    9. quik -vf # to satisfy myself that I've eliminated the bootable
                # flag on partition 4