Does anyone know if I can use my guaranteed seat benefit when booking the following via a travel agent?

DOH (EK) DXB (CX) HKG (CX) YYZ (AA) IAH and returning same way.

I tried searching on the website and the fare is 3X of the fare quoted by the travel agent. The CX segments should be in V fare class, but they are not available from the agent, lowest one that he could do is H or L.

Thanks.