The browser automation guide gives instructions about how to inject javascript, but my understanding is that if a Silverlight application developer has selected to enable windowless mode they have in effect selected to make their application non-accessibile - the javascript invocation is not guaranteed to work (I have only know of it to be a usable option once).
I attach of a Silverlight invoke disable windowless action that I have know to work in a couple of places, and I have known not to work in a number of places. This object should be seen as an example of what you can try but it is not a certified Blue Prism VBO and is therefore not supported by our product team.