It would be nice if someone reports success buying extra "long" flights. So far it's only conjecture that the word "free" in the rules means that extra "paid" long segments will be allowed. I'd anticipate a BA/AA difference of opinion on that, as with their differing interpretation of certain Eastern-bloc fares.

In the context of experimentation, don't forget that if you buy extra segments and then elect to cancel them, iirc you don't get the money back.