Coolant only flows there from the engine when the thermostat is open though... It'd be easier to read engine oil temperature. A common 1/8" pipe thread temperature sender could be installed in the oil pressure access plug (3/8" pipe thread) by tapping a 1/8" pipe thread hole in it, or with a 3/8" NPTF male (1) to 1/8" NPTM (2) female adaptor.
I was also looking at tapping onto the ECU's water temperaure sensor output signal to operate a gauge. According to jh61 who showed the available LED display: "The probe is an NTC. Its value decreases in logarithmic mode when the T increases. Therefore a signal conditioning (with high impedance) is necessary to obtain a linear V/T scale."
Water temperature voltage input is 5 volts as in automotive applications. Voltage output should be between 2.80 and 2.97 volts at 20° C (68° F).
Water temperature sensor resistance:
–20°C (–4°F): 18.80 ±2.37 kΩ
0°C (32°F): About 6.544 kΩ
40°C (104°F): 1.136 ±0.095 kΩ
100°C (212°F): 0.1553 ±0.0070 kΩ