I finished the mod. I know it will work, though I can't test it. The proof is in the math:
3.6/4.2=0.857
Then I attached a 4s LiPo with a resting voltage of 16.36v, hit the switch, and voila:
16.36/18.91=0.865
This corresponds to:
0.865*4.2=3.633
So the A123 will charge to 3.633v/c.
Pics here if you want...