Yes, it is possible to modify decorations in runtime. An example is below.
As you can see, you just need to get a reference to the front panel containing the decoraitions, and then use the Decos[] property to get an array of references to the decorations. These can be passed to a property node.
Unfortunately, the decorations cannot be given a unique identifier, such as a label. So if you have multiple decorations, it can be tricky to modify the properties of a specific one. If you are only dealing with one shape, this won't be an issue. One roundabout way to pick out a particular decoration would be to search through the array references until you find the one with matching size and location on the front panel. Alternatively, you could use a disabled control, such as a boolean push button with no caption, and refer to that directly.
Hope this helps.
-Garrett