Consider a flip-flip wheel and bringing three cogs and a lightweight chainwhip/ Say a 42 x 13, 17 and 23. Start on the 17. Flip at mile 92 to the 23. Unscrew the 23 at the turnaround and screw on the 17. 3 or 4 stops for gear changes in 200 km (?). The turnaround may be a stop anyway.
I am not a randoneur rider but have ridden 130 miles fixed in a day several times. 98 miles with 9800' (158 km with ~3000 m) riding up and down both entrances to Crater Lake and around. (Did that with a 42 x 12, 17 and 23.) I'm headed back to Crater Lake in 2 months and hope to do it on the same bike though now I have a 24 (and am 66; 7 years older). My knees simply won't allow me to do those climbs on my old, one cog. My equally old crotch really doesn't want to do those descents either. But I love riding fixed. So I ride fixed like the early bike racers did; with the ritual of stopping and changing gears. (Oh, when I say flip-flip wheel., that means fixed on both sides as opposed to a flip-flop with a freewheel on one side.)
Ben