Compression is usually legal for behind-wall applications...it's kind of hard to run PEX systems without them. I can't speak for Canada, though.
If it was me setting up a darkroom from scratch...as in, the darkroom is shown complete on the plans (architecture is my other career)...I'd place a floor drain, a couple of sinks, and never bat an eyelash at running PEX. If funds permitted, I might run some bright-polished copper outside the walls, but that's only for the bling factor. And all of that water would be run through a nice filter setup before it hit my sinks. As it is, now, I have to either use distilled water or filter everything I have through a pitcher setup. Don't ask what happened the last time I mixed selenium toner with well water and said "Ah, screw it...the water is probably okay..."