Originally Posted by
heartybob
As per the OP
IB business MAD-LIM
LA economy blue LIM-BSB
LA economy blue BSB-GRU
LA economy blue GRU-MAD
Such an itinerary would probably cost substantially fewer Avios -- but perhaps with a higher carrier surcharge on the IB leg -- if booked as a multi-partner award with BA Avios. And if GRU-MAD is a longer flight distance than MAD-LIM, the entire itinerary might book at BA's multi-partner Economy Class redemption rate, due to a quirk in BAEC's redemption policies. But to be able to book with BA Avios, lowest-tier award seats would need to be available on all segments.
Edited to add: Unfortunately, MAD-LIM is a greater distance than GRU-MAD, so BA would apply the higher Business Class redemption rate to the entire itinerary.