Custom property to drive a value within a linked material

I have a material that has an emission node. I am then either linking or appending that material and adding it to an object in a separate blend file. The object has a custom property that will drive the emission node intensity. Is it possible to have the driver in the emission node be something like parent[“intensity”], that way I can edit the intensity from the object’s properties instead of going back to the blend file that has the material? If so, how would I go about writing that scripted expression?