Ahhh, that's why some lower rad hoses I've seen (non VW) have a metal spring
wound inside the hose.

On an old car the topping off thing would be problematic unless you carry a
spare $40 bottle of coolant with you everywhere.  And what if you have a
slow leak and have to nurse the vehicle 20 or 30 miles down the road, all
the while watching your liquid dollars blowing out the back:)

OTOH, with very low system pressure you may avoid a lot of those pesky
leaks, especially the rubber head seals.

It's been years ago, actually before the internet, but someone wrote an
article about this stuff. I'm reaching into long term memory but the article
did state that the product performed as claimed but they ran into a
unexpected issue - with no pressure in the system, one test vehicle's lower
radiator camped closed at higher RPMs.