Holy crap http://electronics.stackexchange.com is amazing, check this answer out: http://electronics.stackexchange.com/a/132396/1633
(@Jez_Weston I noticed your consideration of fuses in a post about your hat, I’d been thinking about similar things for a future generation of my project…)
There are some amazing people on StackExchange!
Nice! Hey - I’d be curious to see pictures of the inside of your coat for … reasons 
(Doing a similar project, and it sounds like some of the layout/power distribution things i’m thinking about you’ve already nailed)
No problem! It’s in pieces at the moment (the coat is having some pockets for the power buses integrated into it, instead of hanging loose). Hopefully I’ll get it back from my tailor (!) today and get things moving on the weekend, but I can show you the buses themselves.
The great thing about the above SO answer is that I think I already found dedicated power management ICs that do all of the above for a buck or two each. Though the stuff on MOSFET selection is important for that too.
@Robert_Atkins ICs for handling overcurrent / undervoltage lockout, you mean?
Yeah, they handle all the stuff that circuit does, but (of course? This bit wasn’t immediately obvious to me) you need the big meaty MOSFET to actually cut off the battery when it trips.