There are only going to be secondments if the PL pilots are not on the seniority list, otherwise it's transparent commands on seniority (probably about 4-5 years) as opposed to an opaque promotion process on 'merit'. Given that the GSS option in BA is so unpopular do you really think there would be many takers for PL within mainline?
Whilst were at it lets nail the myth that Go was sold off cheaply. BA sold it for many times the value they invested in it. That kind of return would be considered a success in any business. So what if the next owners sold it on again a few years later for more cash? There's nothing wrong with taking the money and running. The only people who slate the sale of Go are those who want to bash BA for their own personal reasons. There was no criticism of the deal from the money men.