I suggest moving this thread over to the ONEWORLD forum where there is a lot of expertise on starting RTWs from IST. But I'll make a few preliminary comments.
Note that BA RTW phone agents do not seem to be interested in setting up itineraries if they will be ticketed outside the country where they are located. In contrast, as mentioned, the AA US RTW desk is willing to set up an intinerary virtually anywhere (at least if the first intercontinental flight is on AA) and you can contact the AA office or GSA in the country where you will start to arrange payment.
Note also that while IST used to be a good starting point, it is now one of the most expensive places in Europe. Depending on the type of RTW ticket you are interested in (class of service, # of continents)- it is likely to be 20% to 50% more than more favourable origins.