Does Weight-Paint work differently in 2.80? I was able to make it work in 2.79, by the usual way.
But in 2.80 it won't react when I am trying to select the mesh (after putting the armature in Pose mode).
Maybe this is still being worked on.

https://blend-exchange.giantcowfilms.com/b/5926/
It seems that was something that had not been worked on yet at that time. It's already been fixed by now.