I don't think asteroids would actually be a good source of Beryllium. IIRC they have approximately the same overall elemental distribution as the earth does, just more evenly distributed - which is great for finding dense materials like gold or rare-earth metals that have sunk towards the earth's core, but not so great for one of the lightest metals in existence.
Gold and the like have sunk into the core not because they are heavy, but because they chemically partitioned into a phase (liquid iron/nickel) that was heavy.
Uranium, which is also heavy, partitions into light silicate minerals, and is enriched in the Earth's continental crust by 3 orders of magnitude above the average of the planet. Without this concentration fission energy would likely never have been considered as an energy source.
It is my understanding that beryllium is concentrated in the Earth's crust by about a factor of 100 over carbonaceous chondrites. Beryllium is actually very rare on a cosmic scale, since it is one of the "X-process" elements made by cosmic ray spallation, not in stars.
