In the UK I belive the addition of a knob to the steering wheel allows the car to be classed as adapted for a disabled person and the rules are relaxed for all other modificiations - anything similar over there in Belgium?
How do you stand with Kit Cars over there, have you tried that route - if the car is a one off (which it is) there must be some way to get it registered.
It does look like if you can get EU papers for the car it should hopefully be easier to import to Belgium
Have a chat with 'SVA imports' in Folkestone Kent - its near the port so if the car has to be moved over here for registration it can be done easily. As far as I know they can sort all the legalities of importing and registering in the UK. (Mention Torquecars when you contact them!) You'll then just have the problem of registering a UK car in Belgium which will hopefully be easier.