Got it! Installing lilo, and having it write the bootsector of sda3, did the trick. Copied that sector to the Windows root where its bootloader would find it, and this time it worked. I guess lilo's bootblock is a lot smarter than extlinux's.
However, I hadn't set a root password, so couldn't log in at first. I've since fixed that, with Parted Magic again, but now I'm realizing I don't have any wifi packages installed, so can't connect to the Internet. Going paddling shortly, so this will have to wait until I get back. But I'm in a fast 64-bit GNU/Linux system now! And I didn't have to overwrite the Windows bootsector!
last updated 2021-03-26 12:23:49. served from tektonic.jcomeau.com