It's called project overgrow. Or at least one version of your idea.
Stoners have been trying to collaborate just this idea since the 60's. So far it hasn't worked, know why? I'll give you a few pointers:
1. Stoners are lazy. You'll probably get 100 people to say they will do it, and 4 will actually do anything.
2. If you get 100 people to tell 20 of their friends to throw 20 seeds, that would make 40,000 seeds. If half of those sprouted that would make 20,000 plants. Animals love to eat the plants, so that would turn it into (at best guess) 5,000 plants. In all likelihood it would be less than that. But if you had 5,000 plants it would take them about 3 months to get to a big enough of a size and maturity to start flowering. By then the cops will find it, and with one quick chop the plant is gone. All your planning gone to waste.
3. How many people do you think you can get to join your movement? Now, how many plants do you think one LEO can chop in a day? Now, how many LEO individuals are in the nation? or the world? My guess, you can't stop LEO. They have you beat in numbers. And in time. Some departments have special task forces designed just to take down mj plants. That means 40 hrs a week looking for dope. How many people are dedicated to plant, and watch, and care for their mj plants for 40 hrs a week?
4. Stoners generally speaking are very greedy as well. If they tossed 20 seeds out, if 5 of them turned female, the desire and urge to go harvest that female plant, instead of letting it repopulate the earth, is stronger than to let it survive.
5. Larger groups for the past 50 years have tried to do this. They failed. What makes your plan so different from theirs? What makes you think you will succeed where they didn't?
I could keep going, but you get the point.
I'm not trying to be cynical or mean, just up front and honest.
I wish it was possible, but it just isn't logical. You have a much better chance of each person planting 20 seeds in their closet, making it so much harder for LEO to prosecute and bust personal growers. If you swamp the legal system you can get the laws changed, but that means that a lot of people will have to put their futures at stake first, which most are not willing to do.
Just a few thoughts.