A workaround exists: on every client root directory, write the following to a file in /etc/kernel/postinst.d/ (you'll probably also want "UMASK=022" in your /etc/initramfs-tools/initramfs.conf, too):
#!/bin/sh
set -e
# Ensure that kernel image is readable by tftpd
vmlinuz_location="$2"
if test -e "$vmlinuz_location"
then
chmod a+r "$vmlinuz_location"
fi
