I was thinking along similar lines albeit BJTs rather than MOSFETs
- it is not for nothing than MOSFETs are often drawn with a parasitic
reverse biased diode.
I've actually done this kind of thing using BJTs in the past although
the intent there was to reduce heat dissipation rather than voltage
drop although that pretty much means the same thing at the end of
the day. The drop is reduced to two collector-emitter losses.
You do need to watch the voltage though since my experience is that
BJTs can breakdown far faster than you might expect when reverse
biased. However that is unlikely to be a problem for battery
powered equipment particularly when you are having one circuit per
cell as here.
Actually, thinking about that I'll have to go through it and see
if the system I used would actually work in that arrangement. It's
just possible the other cells could interfere with the biasing and
I don't have a schematic in front of me to consider that possibility.