Customizing the web UI as what you are looking for from your description is not possible, but you can extend the web UI with widgets which run you custom code, the so called client extension capability.

For completeness, the DNG public API's that are available are:

• RRC Reportable API - creating custom reports to be run from DNG or RPE, read only.
  

• OSLC RM V2 API - the open standard way to programmatically integrate with DOORS Next Generation, there is an elaborate workshop to get you started here OSLC Workshop article. These programs can be from any language that support HTTP REST calls, e.g. Java, C# etc. Hence these are more flexible to run and can read and update DNG data, as part of a web server, or standalone, but have some limitations set by the current OSLC standard, for example module support. In release 6.0.5 extension have been added for modules.
  

• TRS 2.0 - the OSLC Tracked Resource Set open standard REST API to hook into the stream of low level changes done to resources in the DNG application (and other Jazz apps as well); read only.

• and as mentioned above, there is the client extension capability, these are Javascript extension that generally added as widgets you can run from the dashboard, and can be used to view and edit DNG data. It's more tailored to DNG, so has better module support, but is limited to run in a browser/javascript environment.

And there is the private REST API, which you could reverse engineer looking at the requests/responses, but it isn't documented and supported, and can change over time, so usage at your own risk.

 In RTC, we currently add a new Menu entry providing various of our RTC extensions. One of them, the Work Item Bulk Mover, is Open Source. The project also contains the 'Menu Provider' that is responsible for creating a new, custom menu and invokes the custom page our plugin serves. Back in 2014 when we started using this, we were also able to contribute to DNG.