I think Gehard made a very clever solution. Simple and elegant. On the 54 there is actually an overflow pipe as far as I can see. It drains the excess into the slot where the sliding hatch goes. This again has a drain into the bilge (that likes to get clogged, keep an eye on it!). The point is that the capacity of this overflow is not big enough, so it still ends up in the bilge.
I do agree an alarm would be nice. It saves water (expensive on this Island). I was thinking to put some metal in the tip of the level-rod and a Hall sensor trough the wall right next to the scale. Once the tip reaches the sensor an alarm will sound. The nice thing is that the other side of the wall is the electricity cavity, so plenty of power stuff available.