No experience, just some facts from the AIP and the local VFR map:
-) R24A, though obviously Belgian by nature, is inside Paris FIR so must be found in the French AIP
-) R24B and R25 are no higher than 4500 AMSL, which I believe to be the transition altitude, too
-) R24B and R25 are active with EBFN, so normally on weekdays only. Check the NOTAM's, though, one never knows.
Belgocontrol - Operational , section ENR 5.1
Then a personal feeling: Koksijde is military indeed, but sees little action even on weekdays. I think chances for getting cleared are quite good.
Also: the land is flat as a billiard table there, no worries about terrain. One rather tall radio mast though, on the FR/BE border, just south of EBFN CTR, 818". And above all, steer well clear of the French nuclear power plant northeast of Calais.