"From what I've read, their seniority and benefits would be protected if they were to move across."
BA pilots moving to OS under the present offer would lose some pension and not have their benefits protected.
They would not be allowed to move across on mainline scales.
Afaik, Under Balpa's proposals they would keep their pension benefits but would have to accept OS pay & rostering - that would mean that few BA pilts would move across unless it was to take a command at a lower pay rate than they would get from BA. Apart from the command course and possible conversion they would not cost more than a direct entry command.
But it could all be negotiated if BA was willing to negotiate, which they arent.