An old thread, but as I found this while dealing with a similar issue on my own cowl, thought i'd add this for future reference to others...
There are two failures being discussed here:
1. Piano hinge loops failing, in which you're left with a nice aluminum reinforcing strip on your cowl, but no loops through which to run a piano wire.
2. Rivets failing, in which the piano hinge is coming free from the cowling but staying attached to the firewall with the piano wire.
If you used aluminum piano hinge, and solve #2 with screws into the cowl, there's still a chance you'll see #1 some time down the road.
I currently have #1, and will be solving it by replacing the inboard portion of the hinge with a plate riveted to the firewall flange, and #8 screws/nutplates/countersunk washers to hold it to the cowling. I'm still evaluating whether it makes sense to remove the entire piano hinge along the bottom and put in a single long plate... I'm thinking it would be easier to remove 4 or 5 screws along the bottom than to dig past the innermost screw to try and find the piano hinge, as it would be recessed.