In a SharePoint publishing site, what do I need to include in my layout file to make it render the Reusable HTML from a specific item in the Reusable Content list?
In the root of your site collection there is a list called Reusable Content that is used to capture reusable html. If you're editing a masterpage using designer then there isn't a simple >> Insert this reusable text capability.
Instead, insert a dataview pointing to the specific item in the list and including only the Reusable Html field.
Reusable content can be used only one the Publishing page. So custom layout has to include the HTML Publishing content type.
Also pay attention that there's a bug in SP 2010 (On the current time it still exists): If you place a web part within the Page Content control where Reusable content is stored, the last one will turn into the 'a'. Check this question Reusable content disappearing on Publishing Page
Apologies for using an answer for a comment, but I just joined and don't think I can comment yet.
I managed to get my Resuable Content into a page layout in conjunction with this post... Get ReusableHTML from reusable content via Content query webpart
But my Resuable Content is HTML and when it renders it is the source of the HTML, not the parsed HTML. Is this possible? This post suggests not ... Reusable Content does not accept some HTML tags