This probably won't be that helpful Peter, but I had a similar situation with a friend's property in Lethbridge years ago—except it wasn't Duradeck, it was old 2x6s that had some sort of weird outdoor anti-slip material glued on to them. Normally I would have suggested ripping them up and replacing. However, the boards had a sentimental attachment to the owner as they were salvaged from a barn fire that destroyed his parents' building decades earlier. (He said they remind him of his dad.) He wanted angle iron attached to the edges as he routinely dragged things down them, but that would have introduced a tripping hazard as the lip would protrude.
The solution was to use a small laminate trimmer, with a jig that set the depth to 1/8". This allowed me to route a recess into the lip to accept the angle iron. Even though the laminate trimmer is smaller than a full size router you still couldn't get fully into the corners, so these had to be chiselled out by hand. removing the steps wasn't practical but I can't recall the reason why—hence the jig that clamped onto the stair.
I do recall hitting a nail or screw on the first step I did. After that I learned, marked all the existing screws, and either worked around them carefully or countersunk them further. I'll never forget hitting that screw. What a horrible feeling (and sound).
Anyways, this was my solution. Probably won't work in your case, but I guess the take away was to recess any protruding lips on stairs if you add to them.
P.S. I appreciate your comment on Duradeck. Reminds me of my experience with vinyl fences: looks great, but once they need repair, you'll almost always end up scrapping the whole thing. Wood you can sand and refinish at least, and always match paint of course.
Old school baby. (Well, sometimes at least.)
Best of luck.