Though Sagan's suggestion is not workable and of its time as we now know more about Venus, it has an appeal only by way of its elegance. Though such a scheme if workable would require a lot of machinery, the simpler appeal lies in the idea that we dump something that alters the CO2 in the atmosphere and the temperature drops, it rains a lot and we eventually walk around on the surface without burning up.

The idea of a nanoscale influence somehow simultaneously working on the lower atmospheric levels and surface to aid in lowering temperatures while radical changes take place in the upper atmosphere is intriguing. But, if it were workable in any way, I strongly suspect it would be a vast multilayered project pulling in a great many steps and interrelated technologies over a very long time.

I like the recipe analogy like baking a cake. The prep work assures success and then its all put in the oven and we return when its done. The structure, but not the specifics of the Sagan plan has that appeal, as if we do steps A, B, and C then watch from Earth as Venus changes from an totally inhospitable terrestrial planet to an inhospitable terrestrial planet we can land on, (like an earth sized Mars,) then gradually into a more hospitable planet we can eventually live on. But even if such a scheme was workable, its not that easy.

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"Insignificant molehill sometimes more important than conspicuous mountain." - Charlie Chan