Incompatible with Content Profile User Registration

OK - figured it out.
noderelationships uses #after_build to add some CSS classes, on which the javascript that renders the buttons depends.
When content_profile_registration adds the profile node form to the user registration page, it only adds certain elements, notably skipping over some #-prefixed attributes including #after_build.

I'm not sure how to categorize this or which project to file it under.
I'm guessing maintainers for both projects will have arguments as to why it's a bug with the other's module.

For now I will leave it open as a support request and post any workaround that I come up with.