OK; so I took one for the team and ran the gauntlet of phoning QF. After being transferred to someone in the Business Rewards centre the agent took my ABN details and has assured me that it has been added to one of my existing CR bookings. Unfortunately I don't think there is anyway I can confirm whether it has been added.
An interesting point is that the agent stated that only one ABN can be attached to a booking so if I wanted +1's ABN also attached I would have to split the booking. If I'm reading the T&Cs of this offer correctly (and as +1 is also an employee of mine and I of her business), just having the one ABN attached should be sufficient.
I have been planning to make a CR booking anyway so I'll get it done before the offer ends, ring up and add an ABN and hopefully it all goes well.