Hi,
What is a shapekey? sorry, not english:)
I don't think you can simply scale a bunch a vertices like that. You will need to build a small set of tools to achieve this. For example, storing the vertices position in a list, and maybe reference them from your median point, and then apply a scale factor to them, then inject back them vertices position in the mesh itself.
The tuff part will be for you to get the vertice indexes. I can write a little action that would store the vertice list of a mesh into an array, so that you can then work on it right from within playmaker if you want. maintaining another array of indexes, that is the vertice you want to scale. And then an action to inject back the vertice array into the mesh to apply your changes.
Bye,
Jean