I think my biggest concern with the "hard" seam fill options is that they do not give as the deck expands from heat.
I think that moisture variation is more worrisome than heat; the epoxy encapsulation should help with this. But either way if its going to move then its going to stress the epoxy that is bonding it to the substrate as well. I am always thinking about wood movement in my solid wood projects but this being veneer it is a bit more forgiving as Hoodman mentioned:
Which I totally agree with, the planks were screwed which allowed for movement so I think the caulk was necessary at the time as well.I would also think that thin veneers bonded to plywood substrate are going to be much more stable than the planks used in ye olden day.
My veneer is quartersawn and it is a a heavy 1/8", to allow for fairing, but it does still move. I have seen some signs of this where it delaminated slightly on the sheer where the epoxy didn't bond that great to the fiberglass (I have since repaired it). The area that delaminated was indeed in the sun a lot (and I have since added a new shade). I think what happened is that the sun removed moisture from the top of the plank and it started to cup.
If it is fully encapsulated then it should behave much better (not perfect, but better). I guess I should just put my sample piece outside for awhile and see what happens to it