It is relatively easy to rig something to shorten it if you want, in an adjustable way. MacEntrye just posted something on that.
To lengthen it in an adjustable way, I'm guessing that all you really need to do is Mac's trick, but cut the ridgeline inside of the nipper loop, and make the nipper cord longer. Would have the irritating feature of leaving a couple of cut ridgeline strands dangling down though. You could also follow any one of a number of recommendations on HF for a removable/adjustable ridgeline on a Hennessey (which has a similar ridgeline construction).