I don't actually know how this Repeater class works... I tried to play around with it but I'm not familiar with web programming in C#. That said, you can loop through the items in the Repeater... I just don't now how to get them back to a checkbox.
- foreach (RepeaterItem item in myRepeater.Items)
-
{
-
...
-
}
Is there a way to get your checkbox back from a RepeaterItem? If so that should help you.
If nothing else, you should be able to keep track of your CheckBoxes in another list somewhere so that when you click your SelectAll checkbox, you can loop through the list and check them.