It's all theoretical, but I wondered if anyone has applied it to practice here.
If a certain chemical is not harmful via other methods of administration, why would that specific chemical be harmful IV'd? Now I know that you dont want a chunk of mushroom in your vein, but if you could make a pure or semi pure liquid form of the desired chemical, or the chemical diluted in alcohol or water or saline solution, it should work. The point is to get the chemical to your brain, and regardless how you do it, if the chemical hits the brain you will get high.
IV administration happens to be quite different feeling then other methods, so I think it would make for a very interesting experience to IV chemicals commonly used other ways. Imagine slamming mescaline, psilocybin, salvinorin A, THC, etc. People have successfully shot drugs that were in liquid form that normally are done in other ways, such as LSD, PCP, and alcohol.