I managed to use a 2-4-1 to bag two J returns to SYD from Edinburgh, via SIN, by booking 350 days ahead. I travel in April next year. I had to look on BA.com every day, sometimes twice and also had to be quite flexible with dates. By that I mean I wanted to go via SIN and back via BKK but BKK was not available on any date I could use. When I booked on the phone the agent told me they can see availability "a few days more" than is visible on BA.com.
Also, I had to shorten the trip by a couple of days because availablity was not there for every day. As long as you have the flexibility and are prepared to spend time on BA.com, I'm sure you'll get what you want - good luck.
As far as the 2-4-1 is concerned I seem to have been one of the last to get a 6 month extension. Shortly after I extended, it went to 3 months and now it seems is not extendable at all.