Just so you know, I am a criminal defense attorney here in the States, but I know nothing about the law in Canada, so I can't answer anything about that situation.
But here in the U.S., no court would be able to convict you of possession of a banned item without proving to a jury beyond a reasonable doubt that you were in fact in possession of the item at a time after the ban took effect. Even if they could prove that you owned the item at some point in time before the ban, they could not as a practical matter convict you for violating the ban without actually catching you with the guns in your possession, recording you with the guns in your possession, or with very strong eyewitness testimony. In that case, burying or hiding the guns could very well be an effective strategy. If you do this, Owen Sparks' advice about hiding them "off warrant" is very wise.
Remember, the government has the burden of proof -- they have to prove that you DO have the banned gun. You don't have to prove that you DON'T have it. You could tell them a story about how it was lost or stolen, but you don't have to tell them any story at all -- you can just remain silent and let them try to prove that you DID have the gun.
The ease with which a person can avoid getting caught in this way is why governments usually require registration before they engage in a wholesale ban. There are usually requirements that go along with gun registration that make it more difficult to get away with violating the law. For instance, in cities that require gun registration such as D.C. and Chicago, everyone who has a registered gun is required to report when it is sold, lost, or stolen within a certain number of days.
So in one of these jurisdictions, if you told the police that the gun was lost or stolen when they come for it, they may be able to hit you with a violation of the reporting requirement, even if they can't get you for possession of the gun.
Which is why if you wish to avoid gun confiscation, your best bet is to never register your guns in the first place. Then, you are back to the situation where the only thing they can bust you for is actual possession.
You also asked about the possibility of being held indefinitely until you tell the police the whereabouts of your illegal guns. This could not happen in the U.S. because of the 5th Amendment, which says that you can't be forced to incriminate yourself. So if you were asked on a witness stand where your guns are, you could plead the 5th and remain silent. Reporters who are asked on the witness stand to reveal their sources cannot plead the 5th, because they are not being asked to incriminate themselves -- they are being asked to incriminate someone else; so they can be jailed for contempt of court until they comply. See the difference?
Personally, I am of the opinion that "if it's time to bury 'em, it's time to dig 'em up." I do not intend to ever bury one of my guns, I do not ever intend to register one of my guns, and I do not ever intend to turn any of them in for confiscation. Anyone who tries to enforce one of these laws on me will be resisted with deadly force. That is just one of those places where I draw the line.
Let's hope this discussion remains hypothetical... and let's do everything in our power to make sure it never gets to the point where we have to make such hard choices. This would include not only political activism, but also being vocal about the fact that we will not abide by or tolerate these kind of laws. When the people fear the government, there is tyranny. When the government fears the people, there is liberty.