It turned out the reason your server control wouldn't fire in the current project was NOT the fact that you're using Forms authentication; it was an Application Begin Request sub in Global.asax.vb that was killing it. (Something that was inherited and the project didn't even need.)
Sub Application_BeginRequest(ByVal sender As Object, ByVal e As EventArgs)
Try
Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(Request.UserLanguages(0))
Catch
End Try
Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture
CultureInfo.CurrentCulture.NumberFormat.CurrencySymbol = "$"
End Sub
Once it was removed, everything worked smoothly.