Tranquilizers probably wouldn't be all that useful in most applications, unless you are actually getting up there and mainlining the shit. With intramuscular admin they will take quite a bit longer to take effect; not a long time, but more than enough time for someone to freak out about being stuck with a needle.
There's also the fact that you have the possibility of just outright killing the mark outright via overdose. In veterinary medicine, dosages are calculated as precisely as possible and antagonist drugs are carried to be administered in the case of OD. You are basically limited to high potency opioids, which are of course quite dangerous, for everyone involved in their use. Benzodiazepines cannot be used intramuscularly and barbiturate active dosages are too high to be reliable administered through a dart. In veterinary practice, etorphine and high potency fentanyl analogues like carfentanil are typically used. You can't legally obtain those. There are a few analogues that are still not scheduled, but you are liable to get busted under the analogues act, and most of them are on the lower end of potency with active dosages greater than those needed for fentanyl.
The Russains used aerosol fentanyl analogues to try and resolve the Beslan school crisis, and ended up killing a good few people.
I suppose you also have things like chloroform, but I really don't know too much about that, and anyway, if you are close enough to hold something to someone's face, you are close enough to do something a lot more foolproof.