After seeing the corrosion that can develop on these crossbars, I have done something very similar, riveting in a backing plate inside the ends of the crossbars where the bolts pass through. I have used thin aluminum sheet (roughly 0.090" thick) bent to fit the contour of the crossbar and I have also used a broken H17 crossbar as the backing plate material.
The front crossbar is definitely susceptible to cracking and corroding because there are several rivet holes as well as the slot for the dolphin striker all very close to one another. Over time, especially if you sail in salt water, that entire area of the crossbar will crack out.
On the rear crossbar, Hobie changed the size of the stainless steel backing plates that are mounted inside the crossbar ends. Old crossbars have much smaller backing plates (about one square inch) compared to the later boats.
So it sounds like the repair you made was a good one.
The picture below shows one of the plates I made that was riveted inside the front crossbar.
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