I have heard several times a replacement batter using proven technology would weigh about twice the LI battery and be considerably larger. So they can up the strength of the case considerably and still hold a big weight and size advantage. Also ceramic insulating plates inserted between cells to protect adjacent cells from the heat of a cell undergoing thermal meltdown wouldn't add much weight. As they would be similar to the lightweight thermal tiles on the Space Shuttle, but the case will need to be bigger to make space for them.
Also one thing a lot of people are missing is the safer battery that would replace the LI batter has a lot of compromises compared to the LI battery besides weight and size. LI batteries can be charged, and discharged faster without damage. Can survive more discharge recharge cycles, and don't have the memory issues the replacement for LI has. The LI is not just lighter and smaller, it performs at a higher level as well. Which is the reason BA took the risk of going with them despite the known pitfalls.
By the way I don't think the new case will need to be that much heavier to do the job. The old case despite having no venting and major buckling from the pressure inside, likely would have had minor to no buckling if it had been vented. The big difference will come in the way of gas tight flanges for the cover, to insure all the nasty elements from a battery undergoing a catastrophic failure will not be vented into the interior of the aircraft, and that should not add that much weight.