I believe this a problem with the Silverlight plugin and how it interacts with those browsers, and I'm not sure there is currently a work around that we can use. If we don't get the event, we cant react to it. What's the reasoning behind using windowless = true in your application? I believe there are more limitations in Silverlight if you have this option set, so you should make sure there aren't other ways to achieve what you are striving for with windowless mode.

As a work around, if you can grab that event in javascript for those browser types while the mouse is over the map, you could talk over the Silverlight HTML bridge and manually change the window scale of the map by adjusting its properties.