I don't know how efficient it is or if there's a production model yet, but Duke makes a "birdhaus" cab with speakers on three sides, I think. Dispersion in a small space is its raison d'etre. The tweeter on the back of his "acoustic friendly cabs" and the open mid-chambers on his Hathor cabs are cool ideas as well. Even with "regular" cabs (no "extra" holes, chambers, or speakers) there's a lot that can be done with crossover design to ensure good coverage across the frequency range -- by all accounts, that's an area where Duke excels, though I'm sure he's not the only one.