Originally Posted by
ISTFlyer
Actually the OP would have luck from the oneworld rule regarding long-haul premium cabin to short-haul economy connections ( vice-versa ) and should be a admitted into the BA FCO Lounge regardless of status. SAN is longer than 5 hours and in Business Class, FCO is shorter than 5 hours and in Economy Class.
However, can't provide any first hand experience regarding this rule at the BA Lounge in FCO.
The OP isn't connecting between oneworld marketed and operated flights in FCO tho. They are doing that in LHR.