Alright, well I figured out why I wasn't getting much volume through the air vents: the blower fan was installed wrong! (boy, do I hate doing the same job twice. At least it only took me two hours instead of eight...)

I still haven't figured out what the diverter flap is for, from the shop manual it looks like it only opens when the system is on 100% recirc, and lets a greater volume of air through the center vents, but it's closed on any other setting. Since I don't get any air through the center vents period, I think the diverter pod is dead, and the center pod must be disconnected (or otherwise broke) somehow.

I tried looking through the center vvents to see if I could detect any actuation of the center pod, but there's another flap in the way (the one connected to the thumbwheel on the vent itself).

Thanks for your help so far - if you do get pictures (!!!) on the dash removal, I bet a lot of people would love to see them. I know I will.

-anthony