I am working on doing this myself. I will use CPVC which can stand the heat.
I've placed a remote thermometer in my attic and it gets to be 125 degrees
on a day when it is in the 80's outside.

I am going to run say 2" CPVC back and forth in the attic the entire length
of the house. With larger pipe, the water will flow more slowly and have a
longer period to heat-up.

Then I am going to get a solar water heater circulation pump, controller,
and electric solar panel to power it. This will circulate the water into my
hot water heater, but it will sense the temperature of the water from the
attic as well as in the tank and only circulate the water when the attic
water is warmer.

The reason I am using CPVC is I will have to drain the pipes before the 1st
freeze and I don' want any rust. So plastic.

Also I will need to run the pipes so all the water can drain out. And I will
need to install an air escape to get the air out when filling the pipes in
the spring.

So basically the pipes will run back and forth attached to the underside of
the rafters and going higher and higher with a gentle slope (for draining).
Then air release pipe at the top.

And valves for all this to by-pass in winter, drain, and let air escape.

Also CPVC will expand with heat, so need plastic fasteners which allow the
pipe to move back and forth.

Water heater tanks hold heat for a long time. So I am predicting that this
will provide all the hot water I will need. I will basically turn off my hot
water heater and will be able to turn it on only as needed. FYI - I've
noticed that I am happy with shower water being about 102 degrees.