Yeah, I'm fairly certain we did it right - the pads, the message about desecration not popping up, having everyone with all the gems, etc, etc. I've also checked out several vids and can't see anything we did that was wrong. Just he never changed.
Cynara's blog which lays out the strat pretty simply mentions you get a buff called Divine Avenger (+10% damage mitigation, +10% scale, +20% damage multiplier, heals 1 percent every 1 second) when HM is triggered. We were getting that buff, but he wasn't changing. So, I dunno. Just buggy I guess. Oh well.