Thanks for your input.. My saddle rails seem plenty sturdy enough to accept this clamping arrangement. I tend to have the saddle pretty far back on the rails anyway so the rails are starting to curve towards the nose of the saddle where this is clamped.
Yeah this would be a bad arrangement if the rails are likely to bend.
I really wanted this one transverse piece that bridges across the top of the post because, as far as I can see, the lack of this in the original design is what caused them to crack, or at least to be prone to crack, in the first place. Maybe that's OK when you can get replacements from Mr. Campagnolo on a regular basis. As we know, that is no longer the case.
This morning I reduced the height of my 'cradle' by about 3 or 4mm, which makes for easier assembly and allows the bolts to engage with the thread on the far side of the 'nut'. I may reduce it by another mm. later.