OK, I will pitch in for testing, and I will also test the current in my coolant. As much as I ahave enjoyed this discussion though, I think you guys have speculated as much as you can without testing the stuff. Seems to me you have two basic possibilities: the sludge could be derived from the coolant or some chemical reaction that is nothing to do with the HG problem (except to the extent the resulting sludge might block up the rad causing overheating), or it could be symptomatic of the HG problem. I agree we should test it and see.
Long rambling observations follow. Grab a cup of coffee before starting into it.
I do notice that the only two control trucks owned and properly serviced from new (Doug and Dan) both have the sludge problem but have not had the hg problem. That may be a coincidence or a sign that well maintained/treated vehicles from birth are less likely to experience the HG problem even though they can't avoid the sludge. I also notice that Robbie does see the odd truck with a blown HG but no sludge. Again, this raises the question.
Also, Robbie seems to see an unbelieveable number of 80 hg problems even for an 80 specialist. Is it possible this is related to the fact he lives in Colorado? Could it be all those steep grades the local trucks have to negotiate?
I would like to hear what people think of possibility B, which is that the HG is basically a fuse that is an indicator that the engine has been run too hot for too long. What I think is that the damage this causes to the HG may be cumulative over time. THe hg may or may not be designed to blow as consistently as it does, but maybe the fact it goes is simply an indicator that 80s often run too hot breaking down the hg, in which case making a better hg is not the solution, but rather increasing the cooling system performance.
Now it so happens that I just spent 5 days driving through beautiful mountainous British Columbia in my truck crossing several mountain ranges and watching my temp guage like a hawk. It did not move (touching wood) and I used no coolant (touching wood again), but while I was staring at that gauge I did have occasion to think about a few things.
1. A loaded cruiser weighs over 5500lbs. That motor works exceedingly hard to propel it at highway speeds on any kind of a grade. Those are big paint can pistons by modern standards and they must generate an unbelievable amount of heat for that cooling system to disperse.
My question is, seriously, given the weight of the vehicle and the speeds it can manage, is there a cooling system on any passenger vehicle on the road today that has to work harder? As a maybe unfair example, during my highway musing, I could not think of a clear example of a gas powered vehicle on the road where the individual pistons work harder. I think maybe a late model series II trooper or a 4runner with the 4 cylinder might be propelling about the same number of pounds per cylinder, but there is no way they could push the same weight at the speeds on grades that an 80 can manage.
Anyway, I don't know how the capacity and flow rate of the 80 cooling vehicles compares to other vehicles, but I would be very curious to see how they compare.
Now the other observation I have is how easy it is to abuse an 80 when running it up a grade. During my recent sojourn I ran my truck up sustained steep grades for 20km or more with the revs never dropping below 2600 rpm and speed usually holding steady at 60mph and never dropping below 50 mph (My rule was not to run it at over 3000 rpm for a sustained period based on my unscientific experience with i/o boat motors). I think/hope I was taking it fairly easy on those hill climbs. The point is that a new owner might have kept the throttle right down on the long grade to keep up with that stupid Denali that just blew by him. He also is likely to have let his wife, son, or brother in law drive it over the years and they might be even less fastidious. He also might not flush the radiator very often. If they do that they could be overheating it without seeing any indication on the temp gauge and breaking down the HG over time.
2. I can also say that my 80 has the hottest engine bay of any vehicle I have ever owned. It is like opening a furnace door when you pop that hood after a good highway run. I think the engine bay is so crowded it just does not disperse heat. I also wonder if it is intentionally more closed off than other vehicles, maybe to prevent dust infiltration? I do not know if engine bay temp can affect engine temperature or not, but it sure bothers me.
3. Last comments relate to the design limits and decisions on these trucks. It seems to me that Toyota is pushing the envelope a little with the weight to power ratio on these trucks to get them to perform as well as they do on north american highways, and so maybe it is putting stresses that push the envelope on aluminum head technology in a mass production car. I also assume the 80 was designed to be run in relatively stripped down form for 3rd world applications (how much do all the extra seat padding and soundproofing and power accessories weigh) and then adapted for the 1st world environment. I also wonder, did Toyota engineers design the 1fz to have an iron head and then adapt it for an aluminum head? Did they maybe design the 80 cooling system and maybe even the engine compartment venting expecting the vehicle to be operated at lower speeds rather than racing down north american highways at 75 mph all day long?
Anyway, musings off.
For a potential to exist, there must be current and there must be resistance. The coolant (among other components in the electrical path) is providing the resistance and any change in the composition of the coolant that provides for easier or restricted flow of current will be met with a change in potential. 
