In canada it's been ruled that magazines for say 5rds of 50cal in ar platform are legit even if they fit 17 rounds of 5.56 and feed it reliably, because they are designed to hold 5rds

Can we do something similar in ban states?

Sure. Buy normal cap mags, and then proceed to not give a flying fuck.

By who? Canadian law? Canadian court precedent?

I doubt either of those will hold up.

If you have a large cal AR platform and get mags that hold X rounds of that caliber and never load 17 rounds of 5.56 into them you may not go to prison. But if you are caught at the range, or even at home, with mags loaded with 17 rounds in a ban state I wouldn't expect Canadian Courts to come save you in your state...

I bought the incidentally canadian-made cross mag 10/10 i like it

6.5 Grendel in 5.56 NATO mags?

Seems like it hasn't just been long since possible, but perhaps already done in certain ban state markets. But, I wouldn't know for sure as I've been in free country long ever since