Change MovementTrail > Transparency from a global property?


I want to change the “Movement trail > Unselected unit trail transparency” from a button on the toolbar. Perhaps something like changing it to 2 arbitrary values like 10 and 50 as an example.

I tried defining a global property and placing the expression “GetProperty(TrailTransparency)” into the edit field in the Movement trail dialog, but it just reverts the expression to the previous numeric value regardless of what I type.

How can I access the property inside the Movement trail, which is a trait added to most of the counters and cards in my module?