Thanks for the PR! I looked at this together with the companion PR horde/base#136 (this change is only effective in combination with it — nothing in the currently shipped code checks hasCapability('loginparams')).

Mechanically the capability declaration works: Horde_Core_Auth_Application::hasCapability() reads IMP_Application::$auth, and the _bootstrap() logic that strips add/list/remove when no admin config is present doesn't affect it.

However, I think the pair of PRs doesn't fully fix the underlying problem — see my detailed comment on horde/base#136. Short version: with a non-application Horde auth driver, the posted imp_server_key is never consumed. It is only read in IMP_Application::authAuthenticate(), which only runs when IMP's own auth driver authenticates — exactly the driver = 'application' → imp configuration that already worked. Neither the POST-collection paths (login.php, LoginService::processLogin()) nor IMP's transparent auth (IMP_Auth::transparent() → _canAutoLogin() → getAutoLoginServer()) look at the selection, so the dropdown would render but the chosen server would be silently ignored.

The root cause is more likely a small omission in LoginService::buildLoginFormData() (details in the base PR comment). If that is fixed, the existing generic app-auth flow renders the selector and honors the selection, without a new capability or an imp special-case in horde/base.