Bazaar Explorer is easy to extend via it’s accessories architecture. In many cases, adding extensions is simply a matter of creating some xml files. See Extending and Customizing Bazaar Explorer for details.
Many parts of Explorer can be customised via Bazaar plugins. For example, new workspace models can be added to the Initialize dialog using a plugin. You can also provide smarter widgets for key parts of the UI including the location selector and working tree browser. If you need assistance with advanced customisations like this, please contact the developers.