NIMH just has such a short life though. Frustratingly so.
My 1D3 still has the original 2008 battery, and just last week it came home from a 7100 image session at 50%.
The suggestion to use 14650 LIONs would have similar capacity as NiMh. 14500s would have less.
Add 15% loss assuming the suggested regulator circuit is well designed and has 85% efficiency LION solution now has less capacity and you have an extra part to fail.
That's assuming you could somehow run the required balance charge wires inside the pack which only just fits 14mm sized cells to begin with.