I think part of heat tolerance is not needing to drink so much. I know I have had trouble drinking too much on my ill-fated 600k dnfs. Then later in the summer I don't have any problems. I try to avoid dehydration though. As far as electrolytes go, I only use them to balance my intake of water. Or if I'm cramping. But cramping for me is often a sign that I have had too much water and too little electolytes.
Back to the original subject, a lot of randonneurs are of the age where the general population starts to have heart problems, often fatal. We are out, away from services, and if a serious heart issue happens then, the outcomes can be really serious. Fortunately this kind of thing is pretty rare, certainly more rare than among our age cohort that doesn't exercise. I am at risk for heart problems, so I am taking them seriously. It's obvious that cycling isn't a cure for this kind of thing.