How much CO2 is produced by the energy required for crushing the rock?

This is a red herring. Consuming energy doesn't produce CO2, generating the energy does. The answer to "how much CO2 is produced by the energy required to <do action>" is always "as much as your power generation method".

Crush the rocks using solar power, and it's zero.

Still crushing the rock is an activity you'd not do without the goal to bind co2. If the CO2 equivalent of the energy required for this is higher than the amount of CO2 that can be offset by the rock dust, it becomes a CO2 producing activity in total.

If your energy generation process is CO2 friendly you might simply offset other energy use with this energy and generate a better CO2 balance by this, skipping all the rock crushing.

The other question is, what if you do not have the rocks on site?

Transportation of tons of material is not free either. Neither is spreading it on field (labor notwithstanding).

That said, if farmers are already putting tons of imported materials, this could be a matter of switching it up. But if it would be better just to not do this, the whole thing just starts to look silly and worth it only locally at best.

But there's a ton of CO2 in the atmosphere that we'll want to sequester anyway.

This made me think of early civilizations that always sprouted on river valleys. Indus Valley, Nile valley, Danube, Yangtze ..when nomads became pastoralists and then agriculturalists, they must have relied upon water that came down from glaciers or flowed across other lands. River bed silt always has the best soil minerals for growing.

The river bed is full of minerals that it had carried with it..perhaps flowing over different rocks, it brings some with it? Contrast that today..where we have a 45 billion dollar Ag industry in California ..legit Desert.