Just a thought...

Why not plumb (or replumb) a tub with PEX? Should be elastic enough to withstand most freezes (Eliminating/reducing freeze danger/damage change things for us northerners a ton!), not insanely expensive considering (1" is around a buck a foot). Is considered a reliable, long-term, non-leaking solution or copper-substitute for new construction. Widely used in Europe and Asia, it sounds like. And though, obviously, fittings add substantially to the cost, in general PEX fittings are NOT glued (in my opinion a weak link in the PVC/Flex equation). So savings over the (longer) maintenance-free life of a tub?

$1 a foot is not a big cost savings for 1".
Connectors for PEX would be cost prohibitive.
Labor for installation would be cost prohibitive.
Long term reliability has not been proven in a hot tub, subjected to chemistry, vibration, and dynamic pressures that a tub goes through, which is entirely different from household plumbing (which is relatively benign by comparison).

PVC and flex PVC are proven, and as far as the glue being a weak link? Not really. You'd have the same problems with PEX as you would with PVC installation - all it takes is an error in installing a fitting, (glue or mechanical type) - and you'll have a leak. A properly cemented bond has the same strength as the pipe itself.

During a freeze up, typically it is the hard plastic PVC fittings that blow out, leaving the flex intact. Of course there are always exceptions.

Go find a couple of 20 year old tubs and check the pipe/fittings and I can assure you that they are indeed strong, reliable, and will continue to outlast everything under the tub.

Good try though.

(I use pex to replace broken barb fittings on ozone generators, jets and other things - with plastaid or jbweld and it works ok... so far - I'd never want to install this stuff on the output of a 220gpm pump at 15+psi with all the constant vibration going on).