What methods for installing light fixture in middle of room, with lathe and plaster ceiling?
There are two options that don't involve drilling hole in joists and fishing wire:


Carefully cut out an entire lathe. Score both sides of the plaster and ever so gently drop the entire strip.  Notch each joist, run the wire, and install metal cover plates. 
Same as the first one, except use a 2" hole saw at each joist, and remove only the 2" of plaster and wood.  Notch the joist as above. 


Before starting, research the topic of joist notching rules... the middle of a floor joist on the bottom is in tension, and not the best place for a notch.