Solaris 10 Branded Zones broken
In late 2013 or early 2014, not sure when specifically, Solaris 10 Branded Zone support broke in SmartOS. I unfortunately can't confirm whether it is working or broken in illumos-gate as we don't run any vanilla illumos systems.
However given that running Solaris 10 on illumos is a licensing grey area, and given that very few people are using it, it's probably worth ripping out to avoid having to maintain/support it moving forwards.
See here for a brief discussion regarding S10 branded zone debugging in the KPTI work being done by arekinath at Joyent: