TYPO3 Core

Registering plugins based on tt_conent became easy with the introduction of
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(…);

To register FlexForm xml files a similar easy APi should be available:

API to register plugin

\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
    $_EXTKEY,
    'UniqueAction',
    'Speaking title'
);

New API to register FlexForm:

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addFlexFormToPlugin(
    $_EXTKEY,
    'UniqueAction,
    'FILE:EXT:' . $_EXTKEY . '/Configuration/FlexForms/UniqueAction.xml'
);

(Compare the complex definition within http://wiki.typo3.org/Extension_Development,_using_Flexforms#Configure_the_Extension_for_configuration_in_the_BE)