Given that this is a specific benefit that is above and beyond general OW entitlements, I cannot see why BA could not restrict in this way if it wants to

( Oneworld offers no additional allowances for Sapphire status members , so is very simple on that front )

It is not unreasonable imo since it allows the system to calculate allowances and properly inform agents what to permit

Whether this is a BA policy, this would be something to check with customer relations I would suggest