BALPA did the decent thing in the first place and kept positions available in the regions for mainline pilots, which otherwise would have been taken by BACX pilots on inferior terms who would be helping to drive down terms and conditions for the rest of us.
My answer, put the BACX guys on the mainline seniority list as per CityFlyer (with appropriate GF rights). This will allow them to bid onto mainline fleets and those of us who want to fly in the regions to do so without any hassle.
You could even call the new operation British Airways Regional.