Accessing a Label on a dynamic master page?

I have a project that has a master page. On the master page is a label. In the code behind I have a property like this.
Public Property FillInUser() As String         Get             Return lblUser.Text         End Get         Set(ByVal value As String)             lblUser.Text = value             Session("User") = value         End Set     End Property
I would set it from my pages like this.
Master.FillInUser = Session("UserName").ToString
I now have multiple master pages (for Themes) and I set the master page in the page preinit sub of each non master page. and now I can no ...