Thank you - as a data point, I contacted BA as I needed to speak to them about something else, and they confirmed it is only myself who would get the additional weight. I then called Oman Air and they confirmed the same. Additional baggage can be booked on their website (I was told Avios can be used, but can't see an option and I assume is a pretty poor use). I also clarified the additional weight due to the discussion above - 20kg as standard for an avios booking, an additional 15kg for Sindbad Gold/OW Sapphire, to be split across two bags.

It looks like the cost is 29 OMR one way for 10kg, so £120 return pp! I'm still gaining with the avios booking (£100+ avios vs. £500 cash booking), though this will eat into the saving.