with the help of BlackAlpha on another forum, discovered the issue. I thought that briefing.html was not required for arma2, but if you remove that, for some reason the onloadmission won't show. You need briefing.sqf for briefings before the mission, and tasks during, and you need briefing.html for the debriefing.
Not sure why this would cause the onloadmission to fail, but that is what worked for me.