Editing Generic Model Pattern based family instance properties

Hello,

Trying to run a randomizer graph on a set of floor tiles so to randomise their instance material properties.
But Dynamo complaints that the parameter is read-only and I am not being capable of running it while the Mass Editing session is open.

Any ideas, thoughts, first-person experiences on how to tackle this problem?

Sample RVT file also attached.

Project1.rvt (3.2 MB)

Thanks
JUNDER TESTING - PARAMETER - InstanceMaterialRandomiser.dyn (37.4 KB)