I don't know how much a national organization could really accomplish for metal detecting. Most of the members here, seem to want to stay quite, 'fly-under-the radar'. They think it's better to go nighthawking, than talk to anyone outside the hobby. There isn't a huge visible participation level. It's really not going to get much time or attention, at a state or national level, much bigger issues to deal with, besides a apparently small group of hobbyist, wanting to detect protected areas. The only time it becomes a discussion worthy topic at those meetings, is when laws are being broken, somebody's behavior needs corrected. Regardless of organization, or membership, fighting a negative occurrence, isn't going to help much, except to make it appear that is the type of stuff we all want to do, without being hassled over it.
The NRA has always had it's hands full, and they have the constitution to back them up. Every tragic use of a firearm, starts the process of stripping away at a legal right, even though the problem isn't the guns, but the people using them for bad things. Those people will still do bad things, with, or without a gun, they aren't concerned about the laws.
There will always be people in our hobby, that will ignore any rules, laws, or restrictions, and still hunt and loot, any piece of land they choose. Those that simply go online, quick keyword check for 'metal detecting', will only show if there is any potential penalties, if confronted. It's simply risk assessment, is hunting going to be worth any trouble one might get into for being there. These people keep pushing, that if it's not on paper, with penalties, it's a good place to hunt. Yeah, we've got hobbyist who are pushing for tighter legislation, by their actions. What do they care, long as they don't get caught, and the penalties aren't significant, they will still keep doing the same thing, until they can no longer afford to. The rest of us get to pay the long term price, either follow them into dark hunting, or a very limited choice for hunting in public places.
A national organization would only be as strong as the members willing to stand up and be seen and heard. I just don't see the majority of this forum's membership being willing to do that. Paying dues is very minimal participation, membership requires a strong active participation. You need to write letters, make phone calls, attend local meeting (city council), and vote. Money is only a small contribution to the cause.
Let's say one of your local churches needs painting. They bid the job out, looks like about $7,000 to have it professionally done. To me, I see dozens of people in that church, all have done quite a bit of painting over the years, few that do it professionally. The cost of paint, and materials (rollers, drop cloths, brushes), would only be a few hundred dollars. Of course, most churches these days, would go the $7,000 route, just right the check, let someone else handle it...
City parks are the same way, the community could come in, and do the maintenance, plenty of able bodied, skilled people, who would only need to donate a few hours a month. Instead, the city pays a salary to a group of people, to maintain and manage our parks. Lets face it, majority of people would much rather throw money at a problem, than to actually get their hands dirty. They wonder why things aren't they way they want them, forgetting that they are paying someone to make those choices for them. Never bothering to tell anyone, what is is they wanted, or expect. It isn't a tinfoil hat society, where the government reads our minds, and knows what we want or need, then make sure we never get any of those things. If its reasonable, enough people show and interest, they will listen, and accommodate. In most places, there is no harm in detecting, no real reason not to allow the hobby.