Currently, all the ThumbnailDefinition beans are defined in-line in the thumbnailRegistry bean. This makes it very hard to override in the fine-grained manner.
Instead, we should switch to doing what the metadata extractors, content transformers etc do. In this model, we have a parent spring bean for them, which has a reference to the registry. When you create the definition, it will through the parent bean register itself. You can then easily add new definitions.
This is going to be a quick change (couple of hours tops), will make the config cleaner, and will make the lives of people extending Alfresco much easier. However, it will require an entry in the release notes, as people who have currently taken a copy of the thumbnailRegistry bean definition, and overridden it may need to make some slight tweaks (if they have altered the standard definitions etc)
So, one for 4.0, when we can make a change to the config file and add the release note entry