Is it bad practice to have a custom cursor on a website? Not just over a particular tag, but rather everything *{cursor:url();}? How about if you offer a global option to turn it off? Or is this one of those unspoken just don't do issues? ...that I never seem to know about.
Usually I feel if I have to ask about a standard the answer is no, but none of my searches came up with anything specific, nor why it is or isn't considered bad-practice to do so.
I find that "bad practices" usually have some reason why they are bad practices, and also have an alternative way of accomplishing the same thing but in a faster, more understandable, more efficient, or more elegant manner. An example of this might be the general aversion to goto.
For your particular case, I don't know if having a custom cursor is bad practice, but I can't think of a reason why you shouldn't if that's the behavior you want/need. I don't know of a better way to accomplish this either.
That being said, there are general conventions that are followed such as having the arrow as default, text cursor when hovering over text, hand cursor when hovering over links, etc. These conventions are usually followed for convenience and to avoid confusion (my cursor is a hand, so this must be a link). However, if you don't think this will be a problem for you and if the cursor adds to the user experience, then I would encourage you to use it by all means.