Having the sensor in the flow to the engine would be almost as good a spot to measure overall oil temp as the sump. The key is to ensure that the sensor is in the flow and not recessed into an adapter,'t' fitting or banjo bolt.
Re: the S2000
I haven't had mine in 5 years, but if I remember the S2000 oil pan isn't a great candidate for tapping in a sensor..cast, lots of ribbing and it is rather shallow, plus, if I remember, there is a windage tray in it. You can do it, but it won't be the easiest project. A sandwich plate would be better if you have the clearance as there is already a plate for the oil-water cooler.
I would not use one of the sump plug type sensors on a car this low, especially where the plug faces to the side.