Well, it seems I was wrong.
Thanks to a helpful bit of cyber-sleuthing conducted by my Twitter pal Brian (aka iamnotagoomba, aka the guy behind the Japanese 3DS tumblog), I now know that the GameBoy version of Dig Dug was released in Japan--just not as a stand-alone title like it was elsewhere.
Instead, it was included on the second of the three Namco Gallery games that hit Japanese store shelves back in 1996 and 1997.
Each of these portable compilations contained four titles. The first Namco Gallery featured ports of Battle City, Galaga, Mappy and Namco Classic (a golf sim). The second included Dig Dug, Famista 4 (baseball), Galaxian and The Tower of Druaga. The third, Family Tennis, Jantaku Boy (mahjong), Sky Kid and Tower of Babel.
That's even more true when you stick one of the Namco Gallery carts into a Super GameBoy peripheral, as doing so injects each game with various amounts of color while also surrounding them with some pretty fabulous borders (see the screenshots placed throughout this post for evidence).
After dabbling with each of them over the last few days, my favorite so far is the second release--thanks mostly to the "New Dig Dug" mode that's included in Dig Dug.
The others are nice, too, though; especially the colorized versions of Galaga, Mappy (above) and Sky Kid.
See also: 'Four GameBoy titles I'm surprised never saw the light of day in the Land of the Rising Sun'