I found the answer and thought I would report it here as others are probably seeing similar problems. The /proc entries needed to implement the sysctl settings I need are created when the bridge module is installed. That module is installed when br0 is created, much later than boot.sysctl is run so those commands fail. I am seeing that there are many similar things associated with SLES boot system. They will show up as a "/proc/sys/net/ipv6/conf/all/autoconf: No such file or directory" message where the exact file depends on which module is needed. When I added bridge to INITRD_MODULES= in /etc/sysconfig/kernel and rebuilt the initrd with mkinitrd, the modules was loaded earlier and things worked.