This is exactly why I have been putting so much effort into making interspecial hybrid swarms of tomatoes, with hand pollination, before I let them loose outside and start natural pollination. The only exception I made for this was with peruvianum which is self incompatible so I just planted one of each variety and they would have automatically crossed with each other.
Maybe thinking of it as a multigenerational project could help. Especially if you could get others in the local area involved. So even if some peopel move away or their children stay but donât have interest, you could maybe still have enough local trees growing that the community as a whole takes on the project and maybe in a few generations has amazing landrace varieties. Would be a cool legacy too 
I asked some fruit tree folk about grafting, thinking that maybe you could grow seedlings then graft many seedlings to one big old tree. But apparently itâs not so simple, I think they said it depends on the age of the scion, not simply the age of the tree. Iâm not sure if that means it doesnât speed it up at all, but sounded like at least not as much as I was expecting, so far as I remember. Maybe worth looking into more though! But your idea of using the rootstock for vigour sounds interesting, maybe in that regard it could speed up the first fruiting! I guess if you select for taste first like that, then all that pass that test, you could cross and then (or after another round or more) grow then without grafting, for environmental selection?