Thanks @mad!
For anyone else coming here to find a solution, what I basically did was define the variable as normal. For example:
@{ footer-text| markdown }
Then I went to /dashboard > General data and files > Default template settings
I set the default value of the footer text there, and it will show up on all pages that have the footer-text
variable in their page template. You then can edit it on any page, which will replace the default value for that page. However, if you want to change it globally and for all pages you will have to change it from the dashboard.