Coolant Leak - HFH solution

First up, which gen engine are we talking about? Gen 2 is very different to gen 3.
Also you might like to elaborate on your understanding of just what "HFH" actually achieves. What's its purpose?
Same story with other two pipes in that area. What's their purpose?

On gen2 coolant is drawn from inlet side of block close to flywheel end if I remember correctly. It's a long time since I had anything to do with gen2 engine. That coolant flows via a small bore (maybe about 4mm) steel hose, which rusts internally reducing its ability to flow. It then has to negotiate a really poxy 90 degree bend into a flanged joint.
This minimal flow of coolant is depended on to move through the oil cooler efficiently and then goes into the suction side of water pump. I have no idea how high oil temps must climb because of this messy design.
Gen 2 engines are a bit renowned for failing bigend bearings. Definitely to do with oil surge on right hand corners but I suspect high oil temps are at least a contributing factor.

Gen3 the coolant is drawn from the exhaust side of block immediately after the water pump has pushed it into the block. So coolant will be as cool as anywhere in system. It immediately enters the oil cooler via about a 8-10mm rubber hose and then exits into the same sized stainless steel pipe and is drawn along into suction side of water pump. Lots of coolant volume and flow. Efficient oil cooling.
When cruising at highway speeds, up and down mountains etc, oil temp always remained just 10degrees C above coolant temps. So coolant at say 88. Oil would be 98.
Continuous laps on track coolant would rise to 98 and oil to 108 and then stabilize at that. Quite within the capabilities of good quality synthetic engine oils.
Toyota made many changes on the gen 3 engine.
You probably can't alter where the coolant is drawn from the gen 2 block but it's possible to up the size of the hose and use stainless to prevent rust. You should be able to redo the obstruction of that godawfull flanged joint and 90 degree bend into the oil cooler. 
Another significant improvement is to shut off either permanently or with a tap the hose to heater circuit.
This should cause all the suction of the water pump to draw a full flow through the oil cooler circuit.
Once you sit down and know how it actually works you will be able to improve it's ability.
I had one turkey do a "loop the hose trick" in that area and achieved the remarkable result of shutting off the bypass pipe so that the poor unfortunate thermostat had no signal that things were getting warm, warmer, hot, hotter, and bloody hot.

No, gen 3 still has HFH but rarely gives any trouble. Possibly because it carries maybe four or five times more flow and volume keeping it cooler. Seems logical to me. I can't remember exactly what it all looks like in that area but you need the circuit to start from the block via oil cooler and end at suction side of water pump. Some years ago I devised an improvement on a gen2 and "I think" we used gen3 stainless pipes to the water pump and a gen 3 oil cooler and it's base. But it's a long time ago. That car was producing quite good power and on track it certainly fixed the hideous oil temps that he was having prior.
Where are you located?
If local you are welcome to come by or you can phone if that suits better.

A pressure sensor is a good idea though it is best to have an aftermarket ECU that can use the sensor as input to engine protection. 
I have a sensor though haven't installed it yet. I bought mine to detect too much pressure (from a head gasket failure) though it could be used for low pressure failure events too.

I was going to put it in the main hose that comes off of the cooling neck above the gearbox. 
Using a Raspberry Pi to trigger an alarm or a light is a good idea. I recommend a light instead of an alarm because you may not hear it when you are wearing a helmet and the engine is at WOT and 7000rpm 😎😎😎 
As long as your radiator is good, you will be fine. 
I used the large Mishimoto radiator for a long time until it cracked, and I am now using a standard size Fenix radiator. It was very cheap, and I'm happy with it.
If you are worried about cooling and have an old radiator, I highly recommend replacing it with a new one like the Fenix.
I used the Fenix at Mount Panorama last year and the engine didn't overheat even though the ambient temperature was in the mid 30's.