OK... my brains are a little confused.
I thought I had seen such a critter, but then remembered that I had crossed the tracks and it was really over on Processwire where I saw the URL shortener service add-on:
http://processwire.com/talk/topic/1973-url-shortener/
Down further in the thread are some sweet screenshots of the URL shortener in action.
Could something like that be done with MODX then, or is there something out there in the wild?
I would love to be able to use my own domain name 'Crssp.com' and create short links to external content, not content within my MODX site but using MODX backend to generate the short urls. I could then use on Twitter etc.
Thanks for any suggestions on this one.