My touring bike has a Centaur triple crank(30-42-52) with a rear cassette 13-29. I just completed touring Canada and found that I was missing low gearing in certain conditons (mountain passes). Also, I'm leaving for New Zealand and Australia in November.
I don't won't to install a complete Shimano Group Set.
I did a search for Campagnolo granny rings and found they have a 74mm bolt circle diameter. One site (Branfordbikes.com) said they make a 25 tooth ring. That would bring your gear inches down to about 24 from your ~28, which is pretty high IMO. I think the fewest number of teeth for that size of chainring is 24, regardless of brand. (BTW I'm doing my calculations with 700c wheels)
Get a smaller granny ring in 74mm bolt circle diameter. You should be able to go down to 26 or maybe even 24 with your current setup, just make sure your derailleurs have enough "real" capacity, as opposed to actual specified capacity, which is conservative to ease product support headaches. In other words, you'll probably be exceeding the allowed range for your front derailleur if you keep the 52 up front and reduce the size of the granny, but that is not likely a problem. Sheldon sez this should work fine, especially if you add a chain watcher to keep the chain from falling off the inside.
I assume you are using a long-cage rear derailleur so you can drop the inside 30T down to to a 26T and still have it shift well. TA Specialites makes a good selection of these in 74mm.
Also, IRD (www.interlocracing.com) makes a campy-splined 10-speed cassette that goes up to 12-32 which would give you a lower overall gear. Campy doesn't officially support this, but IRD has proven it works without any issues as long as the RD is the long-cage version.
Either one of these would make the difference of a few gears, but both done together would lower your gearing even more.
The only challenge with dropping the front 30T to a 24T is that the range (52/42/24) is a bit much for the front derailleur if you want it to shift and feel the same as your current setup.