Hi Clayton,
Biller Direct or the IS-U version called UCES is using the Login screen mechanism of the UME. So whenever Biller Direct is not finding your login session, the UME service is called. Within the LogonAction in the java code of Biller Direct you find somehwere a method call like "UMFactory.getAuthenticator().forceLoggedInUser(req, res)". That actually will trigger a HTTP redirect internally, so that you get forwarded to the portal logon mask. Once the logon was successful, the portal is redirecting the page back to Biller Direct.
You find the servlet and all JSP code within the app "com.sap.security.core.logon". There is unfortunately no source code available, so there is no chance to modify the app and deploy it using e.g NWDI. To modify the logon mask in a NW 7.0x environment, you only have the option to directly modify the JSP code on the file system. 7.3x however is different, because this system allows you to develop your own logon app and within the UME Setup, you can switch from the SAP default logon app to your custom version.
My company has completely integrated a logon, registration and password reset process into Biller Direct / UCES to be able to provide fully automated, frameless and customized screens, but this is initially very very complicated.
The "content_login_html.jsp" is unfortunately not used at all. I assume a developer did start to implement a logon screen, but it never was finished. There is no way you can re-use that, because there is no Java code behind that does provide any business logic to handle the logon.
I hope that does answer all your questions.
Best regards,
Andreas