greatly reduces the heat destruction of the fruit
It may be a fun experiment but otherwise I don't understand why you are trying to do this.
Marmalade tastes good because the fruit has softened and broken down and partially dissolved in the sugar.
This probably wouldn't happen at your high-altitude boiling temperature.
Also, the marmalade sets to a gel or jelly because of the presence of fruit pectin.
I'm pretty certain that this won't happen at low temperatures.
There is a scholarly paper called SETTING TIME AND SETTING TEMPERATURE OF PECTIN JELLIES published in 1960, but you'll have to pay $12 to read it online.