Yes, I do this all the time. The outbound and return flight will be on different bookings but you will get 50% refund on the Avios for the return flight. One word of advice, make sure you do this through the Avios team (Option 1,1 then 1) otherwise you may have issues. We invariably need to add a shuttle flight and if flying from the US you need to speak to someone who knows how to add an out of date range flight. I had this issue yesterday when the BA phone lines were down and I was put on hold for 40 minutes only to be told there would be a fee to change the flight. I politely said goodbye and called later in the afternoon when the phone system was working again and job done in 5 minutes.