Is it possible to twist bones while posing on Weight Painting mode? On regular pose mode, I can twist the forearm by changing the rotation value under Transform. But I can't use this method on Weight Painting mode, any value I change in Rotation moves the whole mesh. I can still pose the arm by grabbing the control bone, but I can't do twisting motions, just bending.
Of course you can. Just hit R Y Y.

The Transform values in Properties panel refer to the mesh as it is the active object from those 2 selected while Weight Painting.