Display Pro Sites Nag for all Administrators

I would like to be able to display the free trial message to all the administrators on a sub-site, not just the creator of a sub-site.

If we create a site for someone, and set them as admin, then only the super-admin can see the pro-site upgrade message which is kind of pointless.

I am not afraid to get my hands dirty, just want to know where to start.

Thanks
A.