I really have to wonder if any of these solutions would save any money,
even in the long term. Water is typically very very cheap. Electricity
& gas not so cheap. Buying instant electric heaters and what not are
even less cheap. And paying for repairs, even if it is just one leak in
a lifetime, could put you forever in the red on this project.

I have seen claims that the average family wastes 27 gal of water a day
waiting for hot water. I can't even fathom that. We waste, maybe, 2 gal
tops a day. E.g., I wait about 10 seconds before getting in the shower.
When washing dishes (2-3 times a day), I rarely ever wait at all, as
the water isn't that cold to begin with.

On the environmental side, pouring a gallon of water down the drain
once or twice a day might not be such a big deal when compared with
manufacturing a hot water heater, or drilling for gas, or burning coal
for electricity, etc. In many cases, the water will just go right to a
septic or greywater system anyway.

Why not just go get some cheap pipe insulation, put it on your hot
water lines, and see how it works? This is doable for probably less
than $10, and 15 minutes of effort.