I have been trying to book this with a leg from SYD-SIN on flight SQ220. I know that there is a surcharge (of 400USD) for this leg booked in "D". Even though expert flyer and Singapore airlines tell me there is availability in D on a specific day, UA keeps telling me it's not available.

Does anyone know if the *A carriers would see different inventory on this fare? If not, is there a code word or phrase to tell the UA agents so they can book this flight?