Hm, actually I don't see how aligning the pivot would change the object scale? That would just move the object to a different position inside the parent coordinate system.
As for scaling the mesh in one direction, I guess you've seen the
scaleX properties of display objects, right? Those would work on a mesh, too (since meshes are display objects). If you mean to move the actual vertices, the Mesh class has methods for that (
mesh.setVertexPosition()). If you know how the mesh is set up (i.e. the indexes of the vertices you need to move), this gives you full control.
But I'm not sure if this is really what you were after?