Recently I needed to access a control in the master file of a page. Pretty simple but took a little investigation and experimentation.
I was looking for a control in the footer and in the actual application fetch contact information relavant to the current context.
If Me.Form.Parent IsNot Nothing Then
Dim olbl As Label = Me.Form.Parent.FindControl("lblRegFooter")
'
If olbl IsNot Nothing Then
olbl.Text = "Set to what you want, will probably be something dynamically generated or fetched from a database!"
End If
'
End If