[TYPO3-english] Showing only some branches of the category tree in backend-forms (pages, tt_content, tt_news) (TYPO3 6.2., 7.6.x)

[Bert Hiddink [BENDOO e-work solutions]]

Hello,

In order to show only some branches of the category tree in 
backend-forms (pages,tt_content,tt_news), I noticed that:

TCEFORM.pages.categories.config.treeConfig.rootUid = 7

...this works: only cats under catuid=7 are shown.
[tested under TYPO3 6.2., 7.6.x]

However, this does not seem to work:

TCEFORM.pages.categories.config.treeConfig.rootUid = 1,4,7

Only the first cat uid (=1) is shown in the Backend.

Is this a missing feature or bug? Is there a workaround?

Thanks and regards!
Bert