We have an old apricot tree where the trunk sustained unknown damage (It was here when we bought the house, so we don't know what happened to it, specifically) and is mostly hollowed out:
Surprisingly, the tree is still alive and sprouts leaves, flowers, and even some fruit every year.
I came across this idea for planting plants inside an old tree:
I realize that the tree will probably have a substantially shorter life due to the damage, but would filling the cavity with soil and planting plants in it be even worse for the tree? Or won't it make any difference?