Linking position to mask path?

I know I can copy and paste a Mask Path to a layer's Position and vice versa.

But is there a way to link them live with an expression?  I think I'm running into a problem with too many dimensions in an array -- I'm trying to link a Light's 3D position into a 2D Mask Shape (for Particular).   I can copy and paste the motion path & mask shape between them just fine, but pickwhipping gives me an error.