There must be a fair few people who have older Pi computers which they would be willing to donate to good causes and plenty of good causes which could make good use of them.
What I think is needed is someone who could coordinate and administer the collection of Pi's being given away, testing and distribution of those they receive.
Maybe some charity could step forward and provide that service ? Perhaps even providing a FreePost mail service to encourage people to participate.
Maybe someone could think of a charity which is very familiar with the Raspberry Pi and would be well placed to do such a thing