Unless you're in Hong Kong, I'd leave them out of it. You'll ultimately end up dealing with the Cathay office in Japan, so get their email from your local Cathay res number and send them your itinerary. Chances are, they'll get back to you in just a few days with pricing (and payment method/details, if you remember to ask).

You probably know that Cathay doesn't have an rtw desk (at least in North America) - all their agents seem to know their rtw products as well as the specialists on the rtw/atw desks that some of the other carriers have. So the person who picks up will know what you're talking about and the rules including SITI, and won't mind putting you in contact with an office in Japan.

The local office also shouldn't mind checking availability and making bookings for you, but presumably you'll already have taken care of the availability questions with EF or some similar resource and can just email the details with a request for bookings along with your price request.

Cathay did tell me once, when I was trying to get them to book me out of DAM, that they wouldn't ticket an xONEx if they weren't the first long-haul (or some such requirement) carrier. YMMV on that one, but in your situation it's not likely to be an issue.