Talk about timing. I came to this thread to ask the exact same question as the last few above. I'm booked for a rental at OGG for next week through a third party booking my company doesn't usually use. I can view the reservation on the national site, but can't do anything with it. I called the corporate national number and he said they couldn't add my EC to it and I would have to go back to the third party which hasn't exactly been a very responsive resource so far.
I hate to go by the counter, but I hate to give up credit for the rental and of course the EC benefits/upgrade. I'm a little worried about chancing it at the exit booth though. Has anyone successfully done the exit booth association yet?