Is it necessary to fully cure plaster if covering if with a layer of joint compound before paint?
The curing process for lime plaster (Calcium Hydroxide) involves the plaster absorbing and reacting with Carbon Dioxide in the air to form Calcium Carbonate.  

Therefore no air impermeable material should be applied to it for the length of the cure, about 3 months.  Applying joint compound over it will prevent the curing process, with or without paint.

See: http://www.buildnaturally.com/EDucate/Articles/Lime.htm