If you want to have a resource that needs access to a namespaced model, you can overwrite the resource_type_for(model) method from JSONAPI::Resource module.
For example, I have a model placed at app/models/weather/alert.rb, and I created a resource at app/resources/api/v1/weather_alert_resource.rb.
Note that resource is not namespaced.
I extended the resource_type_for(model) in my WeatherAlertResource like this:
class Api::V1::WeatherAlertResource < JSONAPI::Resource
attributes :country, :town, :from, :expires, :description, :alert_type
model_name 'Weather::Alert'
class << self
def resource_type_for(model)
'api/v1/weather_alert'
end
end
end
and now I can make a GET /api/v1/weather_alerts and everything works fine.