[keith k]

Just an update. I made the changes to the center cap retainers to get them to work on the front hubs.
I used a 1.5" hole saw to drill out both plates. While it's only necessary on the plate that goes on the back side of the wheel, I figured the weight reduction would minimize the loads on the screws. The 1.5" hole saw left the hole ever so slightly small, and I ended up doing a bit of filing to get clearance. 1-5/8" might work better.

Some pics below that show how it turned out.
Pic 1: The difference in dust caps; the one on the left is what my car had on it (very big, and would never have enabled the retainer mods to work), and the cap on the right is what I switched to
Pic 2: Close up of the dust caps I used - available at any discount auto chain (at least the ones around me)
Pic 3: The center cap retainer plates before and after the modifications
Pic 4: Outer plate mounted to the cap
Pic 5: Cap mounted to the wheel

There's not a whole lot of margin for where to place the three new screw holes... too far inboard and they don't clear the dust cap, too far outboard and it's very hard to get the nuts on them. The sweet spot is about halfway between the inner and outer edge of the retaining rings, after I cut the hole.