The arguments to the BeforeSetAnyProperty property interceptor need to be less specific. Since the interceptor will be called before setting any property on the entity, the arguments need to be suitable for any property type. There are a number of different arguments which will work, a few would be:
BeforeSetAnyProperty(IPropertyInterceptorArgs args)
BeforeSetAnyProperty(PropertyInterceptorArgs<Engine, object> args)
Check out the BeforeSetAttribute in the DevForce Help Reference installed with the product for more possibilities.