Most joint compounds are simply made from gypsum which is inert.
- Some manufacturers do add other chemicals to make it easier to sand, etc so you may wish to check the MSDS sheets for that brand.
- Once it is dry, by itself it shouldn't pose any risk but it can absorb other chemicals and items until it is sealed.
The bigger issue though is the ceiling itself - you mention deleaded. Does this mean they removed the whole ceiling before or just encapsulated it? If they just encapsulated it, the lead is still there.
Along those lines many older homes have horsehair plaster and/or asbestos in some of the coatings.
My best advice is to remove everything from this room, seal it up, get all the work done, clean & ventilate.
For more information:
Read "I am concerned about lead. I know our house has lots of it." a Q&A answered by Cynthia Phakos.
Also, read "Safety Sunday: The 3 C's of Prevention" a blog post on Sean's company website.