Since you are able to go under and depending on how much height you have to maneuver there, I have the following idea.
Make a form, square, out of2*6, with 1/4" mesh wire, set it on the bottom of the form.
Get a Simpson Strong Tie bracket for concrete to use on a 4x4 post.
Attach a 2x6 under the joists holding the weight.
Cut a piece of 4x4 and attach it to the Joist 2x6 that will support and distribute the weight.
There are angle pieces to attach 4x4's made by SST.
The 4x4 will be "œhanging" over the center of the form with the concrete anchor for the 4x4 attached at the bottom.
Mix some fiber reinforced concrete and pour it inside the form to cover the bottom of the anchoring SST and let it cure. You may use a piece of wood to level the concrete on the form or just leave it as is.