This paper has a few examples of models where it is 93% efficient for mines within 1,000 km. 
Fortunately, there are olivine reserves found all over the planet in a formation called dunite (contains 90% forsterite olivine).
Further, for many of the first beaches, we will be looking to use tailings piles (waste rock) from previously dug and developed mines, as well as the infrastructure from those mines, such as rail for transport. Since olivine is found in volcanic rock formations close to the surface, in the process of mining other minerals that are found in volcanic formations such as diamonds, many tons of olivine rock have already been dug up and deposited in large piles on the surface. By utilizing this rock we would not produce any additional CO2 from mining, and only from crushing/transport.
We are definitely taking the CO2 penalty into account in our calculations and strategy for deployment.
I bet the first few green beaches would just look cool enough to increase he stream of tourists — as long as walking over an olivine beach feels safe and not unpleasant.
Also, is dumping olivine on rocky but flat enough shores an option? That is, may it not replace existing sandy beaches but form new olivine-only beaches?
If you look at the tabletop shaker experiments on the website, the water is cloudy because it is not being refreshed. In an open-system such as on a beach with water constantly refreshing, that would not be an issue.
The olivine can be placed on any shoreline or coastal area. The "tropical shelf-sea beach" set up we constantly refer to is simply the optimal and preferential solution. The main effects we are utilizing the beach for are that (1) the tumbling motion of the waves causes a constant abrasion that breaks up a silica coating that rapidly forms on exposed olivine and (2) the collision of grains on the shoreline causes smaller slivers to chip off, that themselves rapidly weather.
We want shelf-seas because the grains will be pulled off the beach and will continue to be weathered through underwater shear stress forces on the sea bed. Other locations work as well, but the olivine may take longer to weather if there is less motion, colder water, etc.
How do you plan to compensate countries who own the beaches or waters targeted by the project? Do you have an estimate of how receptive a community will be to having their beach turned green, especially if they rely on it for a portion of their income?
A country outside of the world's economic powers might want financial insurance in the event that the the project causes ecological damage and hurts their economy.
Those cities (though not in the ideal location for this project, I guess) would likely be ecstatic to have subsidized assistance (though who subsidizes it?). They're losing the beaches no matter what they do, the question is how long will it take, what will it cost to push it out a few more years, etc. For cities that don't have the budget to dump millions of dollars worth of sand only to have it mostly wash away in the next storm, a green beach is probably much more appealing than no beach.
Most of those beaches are in developing countries. If nourishing the beaches with olivine has some unforeseen, negative ecological consequence, those countries might not be financially equipped to deal with the cleanup. How are those beaches going to be insured?
In the case of California, I'm not familiar with their beach nourishing process, but I assume they are using sand that is more similar in content to what was naturally present. If the beaches have been replenished for years, then we at least have some idea about the short-term effects.
I don't know what the right answer is on this question, but I know that the pain of climate change will be felt by poor nations more than it will be felt by rich ones, no matter what. It may be that staving off climate change, even if it has its own negatives, is less bad than the alternative of doing nothing for those places and communities. But, maybe not. Hopefully it would get a lot of study and small scale experimentation before going big.
What is the Life Cycle Analysis Costs of CO2 Incurred in the Mining, Milling and Transport?
The Life Cycle Analysis (LCA) of the release of CO2 from mining, milling, and transport of olivine creates an approximately 4-6% loss on CO2 removed. We will always work to minimize the transport distance from the source of olivine, and utilize low impact transit such as rail and boats. Further, many tons of olivine are already mined because the deposits are found above other valuable minerals, such as diamonds (found in a rock formation called Kimberlite). Utilizing these piles of waste rock, known in the industry as tailings piles, will allow us to harvest olivine without causing a significant CO2 output. Further, the dust from mining itself can contribute to the offset of the entire mine, as well as the very ground where the olivine is exposed. It starts weathering right away, and many ultramafic mineral mines, abandoned or active, eventually offset their own footprint and even go towards negative emissions. On of our olivine weathering rate sources is actually these tailings piles. See these studies:
Carbon Dioxide Fixation within Mine Wastes of Ultramafic-Hosted Ore Deposits: Examples from the Clinton Creek and Cassiar Chrysotile Deposits, Canada
Integrated Mineral Carbonation of Ultramafic Mine Deposits—A Review
LATERITIC EVOLUTION OF THE JACUPIRANGA ALKALlNE COMPLEX
Koornneef JM, Nieuwlaar E (in prep.) Environmental life cycle assessment of CO 2 sequestration through enhanced weathering of olivine. Working paper, Group Science, Technology and Society, Utrecht University