Yes, that's a snag. One good thing is that (I suspect) unlike Masquerade few people will hone in on exactly the same suspected spot so any digging would be spread out. And anyway I doubt we'll get more than a few people and most will be responsible.
Maybe use an extensive area of heather? You can still dowse but it's no fun digging in it. Or if anyone knows a farmer with some fallow fields? Or how about The Emirates? ;)