Honestly, all the tanking I've been doing - it's been mostly MTing. So being put in an OT position without rage, I rarely know what to do. It's kinda wierd actually.
I do know this:
Battle stance is out, if you've started along the LK gear path, you'll be banking more and more expertise gear which makes overpower not even worth the stance.
Zerker will keep your threat down, if that's important (not really imo, without rage/revenge it's not like you'll be overtaking the MT)
Staying in defensive stance does give you the chance for a salvage should things go bad for the main tank. This is what I usually do, basically sit back, try for some ounce of dps with devistates, SnB procs, (and using shield block to increase the damage) and I swap my revenge button for a 'pickup' macro which smashes taunt, shield wall, and a /rw saying I've picked it up.
In addition, I wear my trash set with the high block value, strength, stamina, and armor. It does slightly increase my dps. However it does feel like you're having a pillowfight with the boss rather then actively participate in killing...