Would be a fancy dance to use physical location as a loophole, I don't think it would hold water unless the letter of the law is NOT about ownership, but is specified as "possession".
Frankly, the "burying my guns" idea is a bit excessive. For what it's worth, it has the same effect as giving the guns up to the gestapo, ultimately, the guns aren't in your hands anymore.
In the event that my guns were ever banned, I'd be going to "speak-easy" route, rather than burying them where nobody could access them. Hidden and accessible is just as effective as hidden and inaccessible.
This would eliminate the need to have them stored long-term. Bury a shipping container in the back 40 or under the shed out back and you can access the weapons for use, maintenance, whatever you want at any time.
If they find the bunker, they'd have found a buried weapons cache, and frankly, we all know how effective our government is at finding hidden weapons stores...
EDIT: I realize that talking about a bunker seems like I'm some whackjob "prepper", but I think any of us that are talking about burying guns and stockpiling ammunition have a little level-headed "prepper" in us anyway. The big reason I favor a hidden storage room, aka 'bunker' over burying them is accessibility. Wouldn't necessarily need to be even a buried bunker, a false wall in a closet, hidden compartments under your floorboards(floating laminate hardwood floors made this WAY too easy to hide these days), etc etc, all are options... Better than burying them where nobody can get to them at all...