This sounds like a phenomenal idea! It's like a living hugulkultur in a way.
just keep it buried, and keep burying lower branches...
and make it into a multi branched shrub/hedge eventually...keep bringing it back to the ground each year before winter.
additionally, the whole *plant the tree at same level as ground* rule of green thumb - doesnt apply to fig. you can just keep mounding dirt/mulch on top of them, keep burying the stems, and mound it up.
it doesnt hurt the stems, the just form new roots where you bury them, and this helps the roots get deeper by adding tons of stuff around them each year.
this makes the roots hardier.