Here's the thing.
I had a G21 45acp for a while and shot my LRN through it with zero leading. As a matter of fact it was much cleaner than the other 45's I've used.
That being said the 9mm round is double the chamber pressure of 45, and leaves the barrel considerably faster. This is a recipe for leading in a poly barrel.
I'm not saying don't do it, but if you do be sure and brush out the barrel every couple mags worth.
On a side note the Glock rifling isn't a true poly design. It's a smoothbore barrel with a few 'speedbumps' thrown in. It's cross section would be a circle, unlike say...my Kahr that would look like a stopsign if cut in half.