I think you have to work your way through the control collections (forgive me if I'm using the wrong phases).  I have a TabContainer which I dynamically add TabPanels to, on each tab I add an UpdatePanel and a PlaceHolder within each UP, in each PH I added a label. 

If I want to change the text of the label I have to do the following (in VB.NET):

Sub AddContent(ByVal strTabID As String)
        Dim tabProcesses As TabContainer = Page.FindControl("tabProcesses")
        Dim TempTab As TabPanel = tabProcesses.FindControl("tab" & strTabID)
        Dim TempUpdatePanel As UpdatePanel = TempTab.FindControl("up" & strTabID)
        Dim TempPlaceHolder As PlaceHolder = TempUpdatePanel.FindControl("ph" & strTabID)
        Dim TempLabel As Label = TempPlaceHolder.FindControl("lbl" & strTabID)
        TempLabel.Text = strTabID
        TempUpdatePanel.Update()
End Sub

As you can see I am  basically finding each control then finding the next within it.

There are probable better ways which I would be keen to see but this works for now,

Daniel