Oddly enough, my lips are the part of my body suffering the most on 400+ rides. I don't have any problem on shorter rides, but they get incredibly dry on longer ones, probably the combination of long exposure to UV light. I try to remember using UV resistant lip balm. As a contingency I always carry Eucerin Acute Lip Balm. Really works wonders when damage is done.
I also carry Ibuprophene and caffeine tablets. After a bad experience with Ibuprophene recently (non-cycling related - but it can really upset your stomach...), I have resolved to use itmore sparingly. But it helped me a lot on PBP, when my legs were on fire after around 350 km. I could barely walk at the controls - 2x 2 Ibu 200s resolved the problem.
I find that caffeine tablets only give me a push for 30 to 60 minutes. This can be crucial when you hit a low at night somewhere where you cannot sleep (for instance because its too cold - happened to my on my first 400 in early may, awful - there may be a risk in the Scottish mountains).
Wet wipes to keep the undercarriage clean are essential. Despite the (in)famous British summer, sun cream as well.
I'll also carry a bivvy bag, an inner liner and most probably a super-light weight Therm-A-Rest to be able to sleep everywhere comfortably.