We take normal dog behaviors and mark them to do it on command. Then, we'll add in something interesting and teach them to do several behaviors at the same time when commanded. (i.e. "Bring it!" turns into "Bring the Remote")
I can see the same process for teaching your dog to dig on command. I see the value in having your dog assist in gardening.
I suppose it would all depend on your ability to mark the behavior so the dog doesn't do it without the command. Like helping you plant a beautiful garden, and then go back out and tear it back up.
Now..this is just me here, but I'd be concerned at whats in the hole that's making your dog so focused on. Snake? Gopher? Skunk? Live wire? Zombie?
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA