Just a thought:
It might be possible to make a solution more "pain-free" by adding a small amount of a local anesthetic such as lidocaine or procaine (though illegal, cocaine is also a local anesthetic- it's used in eye surgery). NOTE that lidocaine and like compounds have profound effects on the heart when used IV.
Also, I could see one's entire ass going numb for some time post injection.
This is just a thought... A lot could go wrong.... But I wonder if such a thing could work?