I'm a little late to the party on this one. We tried filling gaps in plates before, and while the filler material laid ink down...you could clearly see where it was in the print.
We used just plain Alex+Plus caulk to seal plates for a while. It was OK...we found that it also did not adhere to the plate sleeves nearly as well as die sealer.
For full coverage plates...we'd purchase rubber plates bonded to a fiberglass sleeve. They cost like $900 each for our application, but they lasted forever and are a lot more durable than photopolymer.