It's true Columbus Wall has some walls that are opposed to each other on a likely acceptable angle for slacklining... However if I recall correctly I believe there is a substantial height difference between the opposed walls that would make highlining between them pretty problematic.
I wonder if Polly's Cove has some walls that are appropriate?
There are some gaps there, and the height is more in line with what was suggested, ~25' vs the other spots being quite a bit taller. Although as mentioned already, Polly's Cove has been witness to what passes for a NS bolt war not so long ago... Definitely no trees at the top as an alternative. It also sees much more traffic than other known locations.
You're probably better off seeking out other spots that aren't frequented by climbers, the cliffs are just not geographically favorable to highling here. You might even find some suitable rock/trees somewhere crossing the rail way in the south end?!