Webparts in the masterpage are not configurable, meaning that you can indeed add a Content Editor WebPart to your masterpage, but it won't be editable from the browser, as you would like.
What you can do is adding a Xslt list view web part and retrieving fixed content from a list. For example, you can configure the CAML query to retrieve the first item (or item with ID 1) and display the html field wherever you want in your masterpage.
Your users will be able to edit the content by going to the list and editing the item from there.
just a quick how-to (far from being comprehensive, I just want to put you in the right
- create a custom list and add a field of type enhanced rich text.
- In SharePoint Designer, edit your masterpage in advance mode. In Designer mode, insert an empty data view where you want your "dynamic html content" to appear. Then click on add a datasource and point to the custom list of #1.
- click on the Xslt in Designer mode, then on the ribbon List View Tools > Options > Filter and add a filter "field ID equals 1".
- go to the custom list, add a new item (this must be the only item you'll ever create in this list and never delete it!). Edit the richtext field right from the browser, save.
- Your richtext content will show up in every page which inherits your customized materpage.
Todo: customize the Xslt (click on the XSLT webpart, List View Tools > Design > Customize XSLT > Customize entire view). This requires some XSL/XSLT knowledge, for which I suggest you read the excellent Marc Anderson's series Unlocking the Mysteries of the SharePoint Data View Web Part XSL Tags (Xslt LVWP and DVWP are similiar in that both make use of XSLT to display data)