You might have this requirement in the following scenario: your master page needs to display dynamic information (for example Calendar Web Part, Weather Web Part or Twitter which both can be done with XML Viewer Web Part or dynamic HTML from a SharePoint List displayed with CQWB – Content Query Web Part).
This can be easily solved with SharePoint Designer. Please find bellow the full scenario described:
As a first step you need to create a new place holder within the Site’s Master Page. So, in SharePoint Designer open your Publishing Site Collection and then browse to the current master page.