Originally Posted by
Gardyloo
Except watch out for this:
42N . 6. 1 INTERNATIONAL DEPARTURE AND 1 INTERNATIONAL
43N . ARRIVAL FROM/TO THE COUNTRY OF ORIGIN PERMITTED.
44N . EXCEPTION: ORIGINATING USA - 2 PERMITTED WHEN 1
45N . ARRIVAL-DEPARTURE IS A TRANSIT WITHOUT STOPOVER.
46N . NOTE: TRAVEL BETWEEN US AND CANADA IS NOT COUNTED
47N . AS INTERNATIONAL.
Forgot about the US being country of origin (I never buy US/Canada origin OWE tickets, in part because of this rule!).
Gardyloo is quite right, once returning to the US the additional 5 segments are limited to being in the US/Canada and do not allow travel to Mexico, etc. SJU is still part of the US, though.
The circumvention for the OP is to return FCO-LHR-YVR, which would allow the remaining 5 segments to be used anywhere in North America (as described in my original post; having not returned to the country of origin, the quoted rule doesn't apply). YVR is probably more convenient than routing through SEA, in any case! Certainly a nicer flight, and BA has always had one of their better services to YVR (nicer than to LAX, oddly enough).