Align to Mask Expression?

Is there an expression that would allow me to position layers on a random point loosely surrounding a mask? I'm essentially creating a hedge out of leaf particles and I need to be able to dynamically shape the hedge.

I could do something with Particular or another particle based solution but I need absolute control and Particular doesn't seem to have much in the way of controlling already emitted particles so I thought the solution could be to build my own pseudo particle emitter with masks and expressions.

Any ideas?