Hi Jeff.
The problem gets resolved with the patch attached. I commited this change to the githup repository as well. Here is the brief explanation of what was happening. When you have a dynamic tag like <a:span> all of the attributes of that tag are also considered dynamic.
It means <a:span id="xxx"> is in fact gets evaluated as <a:span a:id="xxx">. I designed it this way to save developers from typing so you can just say <a:span if="xxx"> etc. Anyway, what was happening was, it was looking for the attribute id but not looking for a:id in the sub template.
If you want to get this working without applying the patch, there is an alternative way which works fine. Instead of overriding the super template nodes by identifying them with the id attribute, you can use the overrides attribute.
In your example the extension would have, <a:span overrides="#headExtra">. overrides attribute accepts any valid cambridge selector.
Thanks,
Erdinc