Paul;
 I wanted the same thing only I thought the first point of overheating 'sensing' should be the output water temperature itself, rather then waiting for the block to heat up and then sound an alarm. 
 We installed a Borel Water Alarm; 
http://www.borelmfg.com/products_alarm.htm
Simple to install, test  and use. It uses a thermocouple on the engine water output and sounds an alarm when the water temp goes over 200 F.
Bill

I also installed the Borel overtemp alarm, but I do monitor the engine water temp with a Noland Engineering RS11 ( similar products from Actisence and others). Alarms show up on my MFD’s If you put a 1/8 hole in your T-stat bleading will be much easier and you’ll get hot water to the top of the cover where the sensor is. It won’t pass enough water to prevent the engine from getting to operating temp Les

If the alarm's sensor is in the thermostat housing it (obviously) will be monitoring the fresh water loop at the same point as the temp gauge, and I'm not sure there is anything wrong with this.   Its purpose -- as I understand it -- is to overcome the reality that many of us are not eyeballing the temp gauge all that often (particularly on older C36s where the engine panel is located low on the aft end of the port seat). 

But then I'm one of those folks for whom the term "idiot lights" was appropriately devised -- I've been known to overlook (more times than I care to admit) the fuel gauge on my car's dashboard, smack in front of me. 

We installed the Catalina Direct kit maybe five years ago or so, and are glad we did.  I can assure you it definitely reacts quite quickly if you've gotten an overheating problem.