If you mean the routing not available online, this is more like a inherent IT limitations - some possible routings were not shown online at all, while some options shown online won't be able to get the tax/fees quoted. This is common on BA+partner or partner+partner flights and you need to call to get the fees quoted. If the itinerary is not allowed you would have been advised so by agents. It is also a general understanding that the RFS only applies to BA-only booking, not when any partner is added to the booking.

The Long Haul RFS is just a new perk for major subsection of BAEC members in US and UK - bear in mind that before the RFS expansion you were supposed to pay the full tax/surcharges even on BA standalone flights anyway - and this is still the case for BAEC members based outside of US or UK (I would still be quoted a full tax/surcharges even when booking LHR-SAN alone).