I've just been looking at the map. If you assume an U/S transponder, and if you're not going to ask (beg) the BE ATC authorities to transit either the Koksijde CTR or the Oostende CTR, your options are going to be a bit limited.
1. You could go the Southern route, via France. Eg. DVR - Calais at FL60 or so, descend to < 1500' over land, cross into Belgium, avoid all the airspace associated with Koksijde, then proceed via Brugge to COA and enter Dutch airspace <1200'. The part of the Lille TMA around Calais is class E so there appears to be no transponder requirement there (ref Fra AIP ENR 1.6 para 3.1.3).
2. Route to KONAN and from there through the Koksijde CTR, but outside CTR opening hours (mo-fr 9-17, changeable by NOTAM). You'll have to fly < 1500' as the Oostende TMA is class C so requires a Mode S transponder (ref BeLux AIP ENR 1.6 para 1.1). From Koksijde stay low-level and east of the Oostende CTR to COA, to enter Dutch airspace < 1200'.
3. Fly to KONAN as intended, but from there fly to a point just North of the Ostende CTR but South of the Dutch FIR (which is where the NSAA starts at that point). Again, this needs to be done < 1500' due to the transponder requirements in the Oostende TMA. Then DCT COA, descend to 1200' or below, then enter Dutch airspace.
Because of the NSAA you cannot simply route direct from UK into Dutch airspace anywhere, not even at low level. You'll have to go via Belgium, where virtually the whole North Sea coast is controlled airspace down to sea level (except the bit just north of the Oostende CTR), or via France.
Best advice: Get that transponder fixed ASAP. It'll make flight such as this so much easier.