With some constructs, where it is made from separate elements combined, that would technically be possible. However with others no examination of the fare rules would reveal the private commercial revenue share terms of the agreement, etc, on some partnered route, etc. And how addons would be considered relative to that for revenue purposes, even if you could see the addon revenue fares used to give the figure - where the money goes could be different.

I was going to say, you could argue for a phone up arrangement and do it as two things combined, but the online only rule makes perfect logical (business) sense to me too, so I accept it.

Well, actually, given that we now have off line ticketing fees (unless exempt from them), personally I think it should have evolved into a system where the shareholder annual entitlement confirmation activated a flag on your BAEC or BA.com login account, so it could be applied to bookings made by any channel (online / offline) by consulting the system.
But that would be a lot more integration work than what was being used.