This thread would suggest it is possible. However the overland section needs to be shorter than either of the flown section.
http://www.flyertalk.com/forum/briti...-open-jaw.html
This thread also has examples of FT having done this.
http://www.flyertalk.com/forum/briti...l-japan-3.html
I'm planning to do the same thing this year to enable a round the world trip