Can't access custom properties of a linked characters proxy rig I have a rigified character.
I added a custom property to the rig object to drive a shape key on the face. It works as expected in the original file: the custom property slider of the rig drives the shape key on the face.

For ease of iteration, I link the rig (as a collection) into my animation file and make a proxy out of the rig so I can animate it. Animating the rig controls works as desired.

The problem is: the Custom properties of said rig are greyed out and inaccessible in the files where the rig is linked.
This makes it impossible to animate the shape keys for the face.

Is there a way to unlock custom properties in a linked proxy rig ?
if not, is there a workaround people use to animate custom properties on a linked rig ?

Here is a screen shot of the Rig's data panel after it has been linked and proxied. if you look at the bottom, the smirk property I wish to animate is locked. Is there a way to set it up so it stays accessible even when linked, like other properties of the piccolo_rig object are ?
I decided to use a workaround. Being able to use the custom properties would be a slick option as far as UI / user friendliness goes.
But for now I've decided to create one bone per face shape and treat them like sliders where translating the bone in Y axis would drive the face shape.
This works in linked files.
If anyone wants more details about this setup I can write up something.