I wonder if current members could list the seeds they have available for this purpose and list the zone they live in along with the seed list.
When a new member comes along, then new member could be directed to those who have seeds from a matching zone, and the new member could then select say maybe 2 sets of seeds. The current member who has those seeds listed that the new member selects would then voluntarily send the packet to that new member.
As far as in the case of a calamity, then a couple of somebodies would have to be the ones to make that decision and selection based maybe on zone and need.
I dunno, just thinking out loud of some possibilities.
As seeds are reduced from specific current members' supplies, then that member would be responsible for editing his/her post.