Plaster the ceiling

[Nyfiknast]

An acquaintance has bought a split-level house built in 1969. Everything is original for the most part.
He wants to have a smooth ceiling and thought about plastering.
As it looks now, there are rough-sawn boards on the ceiling. Some are a bit uneven.
Do you think it would be possible to screw the uneven boards alternatively sand them to make it sufficiently flat to apply plaster directly, thereby avoiding the whole step of installing a lath panel?

 

[useless]

If the ceiling is high enough, you can cross-brace.
But it might be enough to remove/replace some of the boards that are in the way.