This has an implicit assumption that laws/rules are necessarily
going to come against us. Maybe they will. Maybe they won't. Maybe they'll stay the same. That is an unknown factor. You can say the sky is falling (and maybe it is), but you don't know it, as that's a future unknown. But one thing we DO know (and you will see ample solidarity with me on this in the recent posts bigscoop!) is that:
a) permits and govt. control aren't a good thing (as many people here, and yourself, agree).
b) asking for permits (if in a pre-emptive/pro-active fashion to "head off" supposed certain restrictions) can have the un-desired back-fire effect, of bringing on the very restrictions and attention, you sought to avoid! (in the same way there's ample stories of persons who went for permission, where none was required, and where no prohibitions existed, and got a "no" to their "pressing question", in essence, getting laws, and clarifications, or "policy" created where none existed before)
So if you have one thing that is "known" (that permits aren't good, and are an agreed upon evil), and if you have another thing that is UN-known (that there is necessarilly going to be an out-lawing of metal detecting), then it seems to me that the
lesser of two evils is to NOT pro-act and pre-empt, lest it merely become a self-fulfilling prophecy.
big-scoop, I distinctly recall in the mid 1980s, as the first FMDAC newsletters made the rounds to clubs, that it was the same thing then: "Oh no, act now, coming to a city near you, we're doomed", etc... (how could anyone come to any other conclusion, after reading the scary stories in the letters afterall?). And here we are, nearly 30 yrs. later, and .... whatdayaknow, we can still detect. I know you're going to say
"this time is different".

But all I can say is, that we will certainly have more restrictions if we go looking for sanctions, permits, and archies to love us. That's simply not going to happen. So if it's the lesser of two evils your after, then pre-emptive permits/licensing is not the solution.