While I know that B12 is water-soluble vitamin and (as I understand it) it should be naturally expelled from the body, is this really the case even with continuously high intakes, eg. 1000mcg/day for a couple of months? I would like to know whether there are some known cases of B12 hypervitaminosis and if so, what are the amounts of B12 required for it to happen.
As you said, B12 is highly soluble in water and unused quantity is easily excreted from the body as urine. Therefore, it is nearly impossible to get an overdose of B12, even if you exceed the recommended daily intake.
However, the component which possibly have a very small risk is cyanocobalamin because it releases a tiny amount of cyanide within the body that can bother some people. But note that the amount is so tiny that it is considered medically irrelevant in usual doses.
Injections of high doses of B12 can cause small responses of the immune system as skin irritations, a special form of acne, hot flushes, dizziness and nausea. These reactions are most of the time accounted to the preservative rather than the B12 itself.
Some sources express concern about taking too much B12 in supplements. Others disagree and claim that it's perfectly safe. Google "b12 overdose" and read some articles for yourself, there's too much to effectively quote here. While it's much better to risk the potential rare side effects of taking B12 than to risk having a deficiency, if you don't have a deficiency, don't overdo it. Take at most 100 mcg daily or 2000 mcg weekly. If you do have a deficiency, ask a doctor.