The Ohai docs are incomplete. Here's what I've been able to do so far:

• I've created a custom plugin that adds one piece of node data called "my_custom_data"
• it works when I load it manually in IRB
• I've used the Ohai cookbook to get it loaded on the servers that need it

However, Ohai doesn't load it, neither during Chef runs nor if I run Ohai manually.

The docs, here, are of little use in answering this question. http://docs.opscode.com/ohai.html