It turns out that when I said:
Turns out it is as simple as adding [ValidationPropertyAttribute("ÂText")] to your class definition for the custom control. That's easy!
it really wasn't that easy, at least to get client side support for asp.net validators to work.
After a lot of research, and a bunch of looking through the page output I figured out what must be done.
If you have a custom server control and it is named "MyDateTime1", then the HTML element that you want to check in your client side validation must also be named "MyDateTime1". In my control, I would output a bunch of different elemnts, using things like ClientID + "_textbox" and ClientID + "_Icon" etc... All I had to do was make sure the textbox (which is what I wanted to run the client side validation against) was named simply this.ClientID and presto!