Update...
I had the pleasure of attending the level 2 retreat a few weeks ago. I posed this question of what to do when my spine slumps to Master Lin. He said not to worry about it too much, and that if I notice that my spine has slumped to straighten by drawing my chin back. He even demonstrated the chin tuck for me. He then said that when I build my kidney energy my spine will stay straight all the time, not just during meditation, and without any effort on my part.
Since the retreat I have been going back over the level 1 material. I have noticed that Master Lin does not say to straighten the spine AND draw the chin back, he says to draw the chin back TO straighten the spine. I have been playing with this and find that drawing my chin back does indeed cause my spine to straighten all the way down to my lower back. While this still takes some effort from my back muscles, it is easier than what I was doing before, which was to straighten my spine from the bottom up, then draw the chin back to straigten the last bit at the top. I still have to watch the pelvic tilt separately though.
My two cents. Hope someone finds this useful.