NFHS 9-1-1
The try shall be attempted from within the free-throw semicircle and behind the free-throw line.
I've seen it a few times over the years and my only thought has been, "Why the heck would you want to shoot off-center?"
River Ref wroteI stepped in and advised the defender that for sure let's not have any contact. I was hoping to cover my butt, because I was not certain of how to handle a foul on a free-thrower.
Why even say this? Once you put the idea in the player's head trouble can follow. As for how to handle it, there's nothing in the rule book covering contact with a FT shooter. My first thought would be to invoke 2-3 and call an intentional foul on the defender as well as a FT violation for disconcertion.