This is essentially the same question as posted here: Animating a custom property in a CALayer over a year ago and hasn\'t been answered.
Im creating a custom layer
This has unfortunately been impossible to do with MonoTouch - but we've fixed it for the next beta (5.3.3) which will hopefully be released soon.
Once 5.3.3 has been released you can use this sample: https://github.com/xamarin/monotouch-samples/tree/monotouch-5.4/CustomPropertyAnimation to see how to do it.