A ShowOnTopWhenModal property would have the effect that any modal RadWindow (Modal=true) will increase greatly its z-index, just like it does when it is maximized. This property should take precedence over a maximized RadWindow (i.e. it should increase the z-index to more than 100 000).

A possible workaround:

<style>
    .topWindow {
        z-index: 100001 !important;
    }
</style>

for a markup of:
<telerik:RadWindow ID="topWindow" runat="server"
    Width="500"
    Height="300"
    VisibleOnPageLoad="true"
    CssClass="topWindow"
    Modal="true"></telerik:RadWindow>