Problem updating bootcode on ZFS on root system with MBR

[Thomas Hoffmann]

I am running 11.0-CURRENT (r260850) with zfs on root with MBR.

After upgrading my 10.0-RELEASE (r260669) system to 11.0-CURRENT (r260850)
my zpools reported that they needed to be upgraded. So, I upgraded my
zpools and I am attempting to update the bootcode (as required). I managed
to get the boot1 stage code updated, but cannot get the boot2 stage code
updated. Here is what I have done:

# sysctl kern.geom.debugflags=0x10
kern.geom.debugflags: 0 -> 16

# dd if=/boot/zfsboot of=/tmp/zfsboot1 count=1
1+0 records in
1+0 records out
512 bytes transferred in 0.014996 secs (34142 bytes/sec)

# gpart bootcode -b /tmp/zfsboot1 /dev/ada0s1
bootcode written to ada0s1

# dd if=/boot/zfsboot of=/dev/ada0s1a skip=1 seek=1024
dd: /dev/ada0s1a: Operation not permitted

The final dd statement fails with "operation not permitted". In all my
research,  understood the initial sysctl command I ran would prevent this
particular error from happening.

What do I need to do to get the boot2 code written to /dev/ada0s1a?

Thanks.

-Tom