Hi,
No - I don't think that there is any way to do this. A couple of not-so-perfect workarounds: -
1. Create a presentation and add a text note to a slide which has a fixed extent. The downside to this is that the attribution is pinned to a specific location.
2. Add the disclaimer to the details of the map instead. The downside is that the disclaimer is not always visible when viewing the map
Mike