For most engines, especially the Water Boxer the most damaging failures include the loss of oil pressure, coolant, or coolant flow. If the cooling system is healthy than actual temperature is usually not an urgent issue, unless it causes a failure. For the Vanagon probably the most common cause of an otherwise health system  is the radiator fan failing. If the system is working correctly excursions as high as 240F should be handled without harm. However, loose the pump and localized heating will cause the system to over pressurize often before you see the gauge rise. The easiest warning device would be to detect alternator rotation, off the alternator warning light.

For coolant level you have the coolant level relay. There are a few versions of gauges-operations approach. The earlier vans operate the light and peg the gauge with coolant loss. The later ones will activate the light but allow the gauge to read normal. This at least lets you know what is happening as you get off the road.

As for oil pressure, 86 and later has the oil pressure warning system with buzzer. The downside here is the occasional activation due to low oil pressure even if nothing is actually broken or failed.

Dennis