I have done this successfully via chat. I wanted to book US-AMS-BKK on KLM, and the AMS-BKK flight didn't even have a DL code-share. I wrote a message with the exact KL flight numbers / dates / times that I wanted to book, the fare basis code and price I found on KLM's site, a list of DL e-Credits I wanted to use, and a request to upgrade with GUCs. The first agent immediately transferred me to "another department". The second agent was able to book everything as I requested, apply the credits, upgrade the flights, and reissue the tickets. I certainly got lucky with an agent that knew the request was out of their league and where to transfer the chat, but it is possible, and I have found that writing out all the details of what I want to book in the initial message seems to help.