How to manipulate all user controls with custom property X set to Y?

[Ken Fine]

This supplements a question I asked earlier, about acting on ASP.NET
controls of a particular type.

I'm wondering how to loop through all controls on a page and manipulate
user controls with custom properties set to a specific value -- can someone
offer a code snip? What is the last point in the page lifecycle that I would
still be "permitted" to do this?

Thanks,
-KF