ok, so ya don't want minnows swimming in to get ground up by the pump, so  so some variety of foul-proof intake seems a given.

and the other water filter inside ought to be such that little maintenance is required. ( heck, you can't get people to change their furnace filters.)

so, has anyone done this?  guess a cupro-nickel exhanger would be required.  but getting a low-cost install like this sure is tempting!

and what would happen if the coax gets silted up?  (in spite of your elegant, assiduously-maintained filtration scheme)

Include isolation valves and hose bibbs or boiler drain spigots for flushing.

I wouldn't hesitate to try this, conditions permitting, on a personal property

I'd be loath to risk a customer on it unless they were truly on board for the inevitable monitoring and maintenance.

The volumes of water needed are immense, so the filtration scheme needs high capacity

Can you account for change in lake level such as during flood, drought, or dam release?

has anyone done this?

Yes, you should be able to find numerous reference systems.