This seems to be largely an Apple issue but I wonder if this ought to be posted as an RFE. At minimum couldn't there be a way to use iSync to download the GAL into a local address book, without having to ask Apple to change anything?
Ah, I see this has already been entered:
Bug 30858 - iSync - add sync of GAL
I suggest that this is worth voting for as an interim solution.
These also seem relevant (and I voted for them); not sure if they've been mentioned earlier in this thread:
Bug 33486 - supprot iCal to do attendee lookup Bug 26619 - Include necessary LDAP attributes to provide ical oattendee lookups