Thanks all.
The plan is to come in from Australia to Boston. Using the two intercontinental dep / arr from North America if one is in transit without a stopover from South America to another continent rule, I would stop in Boston for less than 24 hours and continue on to South America.

My current best routing looks like:
Australia - LAX - DFW - BOS - LAX - SCL - LAX - DFW - BOS - DFW - London

The initial North America routing would not involve a stopover of more than 24 hrs. I will be in South America over the New Year holiday period and am more concerned with maximising my mileage in North America after the 1st of the year to work on my status for 2004.

Thanks for all your help.