I'm happy there's someone else ;)
Anyway I'm sure this can be solved, I don't find anywhere the definition of ctrl-l, is one of the few things not inserted in any bundle??
The idea anyway is simply: - get the last line visible on the the screen - get the current line - if last - current < n, then creates n blank lines at the end - go to the new middle
But I don't know how to implement because there are no variables to get the last line...