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.

Hope this helps!