Yeah, you could just have a lookup table that's ASCII characters to "long phrases," and encode the URL that way. Have a bunch of nonsense phrases per character and select them randomly, but as long as the lengthened URL fully encodes the destination URL, there's really no scaling problems to be had. You could even do the whole thing in client-side Javascript if you went that route, purely static site.

Yeah because in theory you could just append the same gibberish string to all the URLs and technically the URL will have been lengthened. Technically you are already lengthening the URL by adding it on top of your domain. Maybe you can base64 it to lengthen it even a bit more and hide the obvious fact that you just added it as a path on top of your domain.