While you wouldn't be allowed to call a maple-cultivar 'Aureum', there would be nothing stopping you from publishing new forms (forma) of other yellow-leaved maples, e.g. describing Acer platanoides 'Princeton Gold' as Acer platanoides f. aureum 'Princeton Gold'. I suspect that's how the examples you listed originally got their names? I'm guessing the applicants simply didn't know, they did choose an invalid name twice after all... One would have to read the Canadian PBR code, but it would make sense for it to require cultivars having a new and unique name, whomever approved the name 'Borealis' could have followed the Canadian regulations to the letter, unaware of the ICNCP. It's an interesting discussion. Still, with so many Latin words being used in living languages I'd say using them defeats the intent of the code. What if someone discovered a northern form of Lonicea caerulea and named it var. borealis?