Do you mean dynamic controls created from <Instance> elements, or sub-contexts added via <LuaContext> elements?
The former you can't lookup (as they don't have unique ids), the latter can be found as normal (but IIRC, there is some oddity that the ID attribute in the <LuaContext> tag must be the same as the ID attribute in the corresponding <Context> tag).
Unless you're trying to get something in a core stack (eg one of the top panel controls) you shouldn't need to lookup dynamic instances as you can store references to them as you create them.