Just back on this to leave something to posterities in case other hit this issue.
Although the approach above works, there is a drawbacks on modifying the graphics.outlineColor in this way: it seems to trigger a full repaint of the feature.
The issue is similar to one I already faced on that thread.
On that thread I was trying something very similar (modifying a GeoJSON color) and the solution was to use CallBackProperty.
This is not working with outlineColor or I fall again in the original issue: Cesium doesn’t complain, but the color is just white.
So: there’s something a bit weird here.