Currently, the 'Register' link is static with an href that sends the users to an Apereo website.
A welcomed improvement is to make the link href value configurable as a portlet preference. Another nice-to-have is make the entire link an option driven by a portlet preference.
The Login portlet is defined in https://github.com/Jasig/uPortal/blob/master/uPortal-webapp/src/main/webapp/WEB-INF/portlet.xml#L834 . There, these new portlet preferences can be added.
With EL, accessing preferences is easy. See https://github.com/Jasig/uPortal/blob/master/uPortal-webapp/src/main/webapp/WEB-INF/jsp/Invoker/logout.jsp#L23 for an example of pulling a value from preferences.