I've done some more investigating recently into this issue that I'll report here in case anyone else runs into it. The first thing I did was contact Apple support. Their answer was essentially, "Sorry, we don't have that information about how our product works, maybe you should look for support on the Exchange side of the problem." However, I'm fairly sure Microsoft won't be able to help me figure out what permissions iCal needs to allow calendar delegate reviewing.
So I fired up pfdavadmin to see what permissions were and weren't allowing me to open calendars in iCal vs. Outlook. It turns out that the calendars I could open in Outlook but not in iCal gave reviewer permission to \Everyone (or in Outlook-speak "default" was a reviewer). Only when I was given permission individually could I open a calendar in iCal.
Moral of the story, iCal and/or EWS doesn't treat the \Everyone or default permissions the same way Outlook does. Oh well.
