I figured out a shell one-liner to do it:

ls /var/lib/initramfs-tools | 

    sudo xargs -n1 /usr/lib/dkms/dkms_autoinstaller start

This works because the names of the directories in /var/lib/initramfs-tools are exactly the kernel version names that you need to pass to dkms_autoinstaller to tell it to rebuild all modules for those kernel versions. Note that if you have uninstalled some old kernels, their directories might still be lying around and cause some errors to be reported, but this isn't a problem because dkms_autoinstaller will just do nothing for those kernel versions that aren't installed.