TYPO3 Core

The reason we didn't implement this: 'itemsProcFunc' is still the ugly child of formEngine / dataHandler.

Especially formEngine has the much more powerful FormDataProvider solution to manipulate things like these.
The valuePicker is a client-side thing only, DataHandler does not fiddle with it. As such, you can already reach the same by adding some formDataProvider that manipulates the items, which gives you the advantage that a lot of related data is available, for more than is given to itemsProcFunc.

Do you think that would be fine with you?