Racket allows for macro/exec, but it strictly controls the namespace for both of these things. The former mainly via hygienic macros and the latter via requiring exec to be given a namespace to execute in.
I think allowing things like NSFunctionExpressions are ok, but you need to have an explicit namespace, and the default needs to be an empty namespace, not the current/full namespace. Having a default is a good idea because if you don't add one as an API designer, other people creating wrappers around your library will, so at least try to make the default safe. It sounds like that is somewhat similar to your idea of registering a function.
I think allowing things like NSFunctionExpressions are ok, but you need to have an explicit namespace, and the default needs to be an empty namespace, not the current/full namespace. Having a default is a good idea because if you don't add one as an API designer, other people creating wrappers around your library will, so at least try to make the default safe. It sounds like that is somewhat similar to your idea of registering a function.