NT- I think you will find the BA/Balpa agreement is for 50% (as per GSS) of all new OS commands to go to BA mainline pilots. The L'avion deal has complicated matters somewhat given OS now operate under a French AOC, however the BACC is currently thrashing out the fine details. I understand OS management did put up a bit of a fight, but with the full backing of WW the first mainline pilots should be joining soon.
There is definitely no recipricol agreement and I don't ever see this changing.