@ScottBradley
If you know size of the control you need to position that's not hard and you can use sample from mentioned thread. But if you don't know size of that control I see only one way - declare same size invisible control before your real control and position your real control as relative to that invisible control (I don't like this way but it works). You can see same trick in Xamarin.Forms samples (FormsGallery\RelativeLayoutDemoPage.cs).