TYPO3 Core

Which TYPO3 version are you using?
According to the feature request you reference this should already be possible

I performed a test with the latest 8.2 master; as far as I can see:

TCEFORM.tt_content.imageorient {
  addItems.100 = Value
  addItems.100.icon = fileadmin/icon.png
}

works, while

TCEFORM.tt_content.imageorient {
  altIcons.26 = fileadmin/icon.png
}

Further notes:

- The online documentation https://docs.typo3.org/typo3cms/TSconfigReference/latest/PageTsconfig/TCEform/Index.html does not mention altIcons
- the changelog .rst file (/typo3/sysext/core/Documentation/Changelog/master/Feature-35891-AddTCAItemsWithIconsViaPageTSConfig.rst) reads as example:

TCEFORM.pages.doktype.addItems {
        13 = My Label
        13.icon = EXT:t3skin/icons/gfx/i/pages.gif
    }
TCEFORM.pages.doktype.altIcons {
        123 = EXT:myext/icon.gif
    }

But the altIcons property only works on the following condition:

"config type=select, maxitems > 1, renderType=selectMultipleSideBySide set" 

(thanks to W.Wolters and S.Moog that helped me digging the code)

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68776

Applied in changeset 881906ee61817ded8f942a34636b55bfd9c87562.