Not sure I understood exactly what you mean, but if you use click on button1 for BG1, in the click function you could write:
mySO.myBgFrame = 1;
mySO.flush ();
and than, when you need to set the saved BG:
if (mySO.myBgFrame){
bgMovieClip.gotoAndStop(mySO.myBgFrame);
}
Is this what you need?