In my opinion, the job will take a lot longer to do from below and you will not have a guarantee of success. The only way I see it being reasonably easy to do from below is if you cut the liner out in one piece or large pieces and re-use it. This will give you something to prop up against the balsa core. I can't see glassing overhead, trying to hold resin wet biaxial cloth up in multiple areas. The balsa or other core can be pushed up against thickened epoxy against the inside of the outer skin and should stick fairly well. Then wet out the core material with epoxy that is not thickened. The inner panel coated with a a good mix of thickened epoxy then is put in place and you shore it in place with plywood panels or something similar and sticks braced to whatever. Cut your braces in advance and be sure to have enough to take the place of dozens of hands - you'll need them. After the epoxy has hardened remove the bracing and grind the edges to a taper and apply biaxial cloth to finish it off. If you would like to see pictures of core replacement go to The Plastic Classic Forum :: Index
and search recoring decks and you will see pictures of various projects with descriptions. Many plastic classic members have done this, although almost all from above deck. I will be doing some areas of the deck when the weather is warmer but from above. Good luck.