Warning: I know just enough to be dangerous, and nothing Campy specific.

It seems you could just get another crank and swap them back and forth. Pricey and would involve some derailleur and chain tweaking potentially, but should be doable.

Another option may just be to get a different (smaller) small chain ring to give you a lower low. You'd still have to uninstall the cranks, replace the ring, and re-install. Royal pain, but cheaper.

Final option would be to just replace the cassette with something lower. Probably the easiest thing to do. Either get a spare rear wheel to use with the lower cassette, or just mount and unmount. You'll have to adjust the derailleur as well likely, and the derailleur may limit how big (easy) you can go on the rear (ie: you probably won't be able to use a mountain bike cassette with a road derailleur).