This is a workaround with an alternative, pending proper answer.
Wbar - a light dock with few options (fewer than even the lightest out there like Plank and Docky) is installed by default in Manjaro-Fluxbox 0.8.13.1.
It is meant as a simple application launcher but it can also show taskbar with open windows icons, and thus work like iconbar.


So, the workaround is to use this feature and replace the iconbar.
As Wbar is normally hidden under windows, it should be positioned just like the toolbar (up or bottom) and the toolbar height should be increased so that the Wbar is always visible.
The toolbar changes are made by editing ~/.flucbox/init:
To remove iconbar:
session.screen0.toolbar.tools: systemtray, clock
To increase toolbar height:
session.screen0.toolbar.height: 40
To shorten the toolbar and make place for the wbar:
session.screen0.toolbar.widthPercent: 25
To set the toolbar to bottom right, while wbar is placed bottom left:
session.screen0.toolbar.placement: BottomRight
To increase the toolbar height, so that it matches the dimension of wbar icons (if these are set at about 30 in Wbar's Settings-Effects):
session.screen0.toolbar.height: 40
Empty spaces in the wbar can be created with fake commands like zzzzzz and an edited png to look empty. (Open a png icon in an image editor, use the select tool to select all image there and delete it; then, save as png.) The empty space can be enlarged by adding multiple such ghost commands, and in this way the taskbar can also be centered if so desired. If no launchers are needed, they can all be removed from Wbar.
Considering the toolbar, if it is not needed, it should not be disabled completely (by editing the init file and changing session.screen0.toolbar.visible), or the wbar will be hidden when windows are maximized; instead, the toolbar width should be set to 1 at session.screen0.toolbar.widthPercent:.)
It is possible to add keyboard layout switch icons to the Wbar as indicated here.

~click image to enlarge~
Often enough, Wbar needs to be refreshed to avoid some delayed response. Just right click it.