Thanks Perche, I asked the concierge and he wrote back:

"In case of strike, it depends if all of them will do that , will find a taxi boat ."

Which I don't entirely understand but take it to mean that they will figure out a way to accommodate us regardless of the strike. Their Riva boat is running until the 23rd btw; I think they must consider that the start of the cold season since it gets docked until the spring. They typically only offer it for either set or custom tours of the lagoon and islands, so it's not regularly used as a means of private transit but I'm sure they'll take it out if needed. As soon as we arrive, I'll be sure to ask about the strike and find out whether the boat is free for our lunch reservation.