AFAIK the issue with using antifreeze is that it's mostly glycol (propylene or etheline) with an anti corrosion package and (usually) an anti-micro-boil package (water wetter, surfactant, however you want to think about it).
Glycol has a lower specific heat than water, so it can carry fewer BTUs per gallon (or however you want to think about thermal units and volume). If you put in little enough that there's no problem with specific heat, you may not be adding enough of the anti corrosion package.
I'm unclear as to whether the water wetter does anything positive or negative - I'm pretty sure we're not going to be having problems with micro boiling even without it.
There
are automotive cooling products designed without glycol (glycol is incredibly slippery and it's pretty much banned from any vehicle on the race track because a spill could cause accidents days later at the next rain even though it seemed to be cleaned up). These products
do include water wetter as racing engines are certainly hot enough to cause micro boiling - but otherwise, they're pretty much just anti-corrosion. I've been using Zerex (now part of Valvoline) "Racing Coolant" for a few years, in a number of w/c'ed PCs (mixed metals - Swiftech blocks - but these are hard anodized) with no problems.
Oh - it's a pinkish color, which you may not like. Automotive coolant additives are color coded for reasons that don't matter here...
Finally - I don't
think automotive coolant additive packages include any kind of anti-algae - just not an issue there (or maybe the glycol is toxic to algae). I would have expected the 100C temps to do the trick, though...
