Ok, like I said, I'm new to writing server side controls.  I figured out I can add an existing server side control to make a "compound" control instead of writting all the HTML directly.  So I just added an ASP.NET text box and then called the normal InvokePopupCal methods to build my javascript call.

I had problems getting the PopupCalendar.js include registered, I had to put the RegisterClientScriptInclude call in the PreRender event.  So here are the relevant code bits that I have so far.  Still need to fully test this but I think I got the core bits working.

 
Private popupCal As ImagePrivate dateBox As TextBox

...

Protected Overrides Sub RenderEditMode(ByVal writer As System.Web.UI.HtmlTextWriter)
	'Add the date data entry box
	dateBox = New TextBox
	dateBox.ID = Me.Name
	dateBox.EnableViewState = True
	dateBox.Text = StringValue
	Me.Controls.Add(dateBox)
	dateBox.RenderControl(writer)

	'Add the calendar control
	Dim attrib As String
	popupCal = New Image
	popupCal.ID = "imgCal"
	popupCal.ImageUrl = "~/images/SmallCalendar.gif"
	Me.Controls.Add(popupCal)
	attrib = CType(DotNetNuke.Common.Utilities.Calendar.InvokePopupCal(dateBox), String)
	popupCal.Attributes.Add("onClick", attrib)
	popupCal.RenderControl(writer)
End Sub

 
	If Not Page.ClientScript.IsClientScriptBlockRegistered("PopupCalendar") Then
		Me.Page.ClientScript.RegisterClientScriptInclude("PopupCalendar", ClientAPI.ScriptPath & "PopupCalendar.js")
	End If
End SubPrivate Sub DateEditControl_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreRender