Overview

Today, the custom Javascript feature isn't included in WF due to potential security risks from untrusted users. An admin of their own instance can be considered trusted, so they should be able to add custom Javascript to their blog.

Implementation

If configured as single-user instance:
Just like on Write.as, include a Custom Javascript section on the blog's customize page, below Custom CSS. Include the same description:

This will run on all pages — your blog index and individual posts.

Below that, include the Ace editor set to Javascript mode. The field name for this data should be script.

This should be all the work needed -- all the backend plumbing, it appears, already exists for handling the data.