OK, if I understand the problem correctly, it sounds like there are several themes being used by customers that don't fully support Confluence 2.8. The best solution would be that these customers stick with themes that are designed to work with Confluence 2.8.
I don't think it's a good idea for us to switch back to using the old theme system in our plugin... we want to support the newest features as a general rule.
I propose we add a new web item plugin that works with older themes. We would ship this plugin disabled by default. The name of this web item plugin might be: Web Item to show Add Diagram link (2.8+, legacy theme support)
If a customer wants to use themes that haven't been designed to work with Confluence 2.8, they can disable the 2.8+ web item, and enable the 2.8+, legacy theme support plugin.
How does that sound?
I created a confluence issue here: http://jira.atlassian.com/browse/CONF-12235