Ray - you are correct about the precedent, and there is indeed a lot of switching. LGW operated on inferior pay and rostering compared with LHR for years. When the base grew more successful/profitable pay moved towards that of LHR, to the point that it is now identical but for a pay cap after 16 years at LGW. Rostering is still different but over time that may change too.
When a LGW pilot moved to LHR they always moved to the LHR mainline pay scales and vice versa. That would be the case with OS too. If a mainline pilot wished to move to OS they would move onto whatever pay scale was agreed between BA and the BACC, which would probably look very similar to the current proposed secondee T&Cs.
The system is complex but not necessarily a problem in BA. All pay negotiations in BA needed to be shrouded in mystery and concealed by smoke and mirrors lest anyone believe you have achieved anything other than the corporate offer. The complexity of different bases, rostering rules and working arrangements provides a suitable smoke screen to prevent the other unions in BA from seeing what is actually going on. The BACC have very detailed information on who is earning what and where and what the overall pot looks like before and after. Open Skies will run under it's own CC and will be responsible for it's own affairs, although with their pilots on the master seniority list there would undoubtedly be close links to the BACC.