I’ve just started looking into this myself, but in your review did you see this thread on Adding a custom transform? Someone in that thread ran into the same issue.
It may be silly, but it looks like in all the examples the function name starts with an underscore - did you end up trying that? (i.e., naming it _do_crop). Also - do the transforms you pass to ImageDataBunch need to be in a list (per Sylvain’s comment in that thread)?
If you figure out the issue could you post the solution here? I am sure I will be running into the same issues as I look into this.