The white pipe in the photo carries graywater from the house and deposits into the tub.
I dug around three sides of the tub, exposing the upper edge, and removed 2-3" of dirt and gravel on the inside surface. I wiped that 2-3" of tub edge clean, then squeezed a bead of PL Roof & Flashing Sealant onto the cleaned edge. I cut some scrap pond liner (left over from our ecoroof projects) into approximate 3 1/2" - 4" tall strips, and pressed it into the sealant bead to effectively raise the waterproof edge of the tub by about 2 1/2" on three sides. I did not raise the fourth side, where the water is supposed to overflow.
After the sealant cures overnight, I'll refill the interior of the new pond liner extension with gravel, and bring dirt back up to the outside edge of the pond liner. This should prevent the worst of the "wicking" and allow water to overflow as originally planned.