I used to worry about stuff like this when doing long overseas bike trips, and the legacy bike I'm riding out the door tomorrow on a vaguely open ended trip kind of reflects that.
[img] 
[/img]
Bar end shifter could be set to friction only if everything gets knocked out of whack (but it never happened). Sliding dropouts would make a ss conversion straightforward (it's never been necessary). The frame, a steel Lava Dome bought off here for £60, is 26" but is currently happily taking 27.5" cup'n'cone wheels (also bought for £60 off here); so I know I can crowbar in various wheel and tyre sizes (never needed to do that). The forks have canti studs (and the front rim I used to use, Rhyno Lite, had a braking rim) so I could've substituted Vs/cantis in the event of total failure of front and rear disks (guess what? Never happened).
These days I'm less comfortable with burning through a load of carbon traveling to a poor country to use it as a backdrop for my ego, but if I was I'd take a decent tyre boot, gaffer tape, a couple of hose clips and some commonly sized bolts. That's all I needed to sort the problems I did have: sheared rack mounts, broken saddle clamp bolts, etc. Pack the bike well for travel on the way out, that's where it's most likely to suffer trip ending damage on a three week holiday.