If you have no other options, I would most definately leave a note in plain sight to remind the contractors to put up the piece of drywall. I deal with sub contractors every day and know that you can not rely on them to remember something like that
Not because they can't remember things but because they are so busy. Make sure on the note you also remind them not to let the kittys out. I would double check the chemicals also. I don't know what sort of work you are having done, but you want to make sure with the contractors. Also, are the walls being painted? Wallpaper removed by chemical? I am sure you have all ready thought of these things, but just wanted to mention to be on the safe side. The kittys should not be around to breathe any of that in.
I think the best thing, if any way possible is to close them in a seperate room. That way they are safe from getting out, the dust, and they may even feel more secure. I am sure they will be stressed out with all the noise. Maybe you could even get a Feliway plug in for the room you close them in.
I am sure you will do everything you can to keep them safe, I will say a prayer that they are ok while you are gone