Look, I’m new here, but I think you need to add it in an xml file in app/etc/modules to use it on all pages. I have a file called SFA_All.xml (SFA is my namespace) that does this for my custom attributes as well as adding modules, etc. This is the format for adding a custom product attribute:
I hope that is what you are asking (or at least helps). Good luck.
EDIT: My (admittedly limited) understanding is that all of the xml files in this folder get loaded, so that you can place that code in one and name it anything (.xml) and it should work.