I would not invest work into Qt5X11Extras, as it depends way too much on X11, for which a port exists, but it cannot draw on the screen at all, it is useful only for running X11 programs locally while displaying it on X11 capable remote host.
The related parts of OBS should be rewritten for Haiku, or the parts depending on Qt5X11Extras could be disabled, but god knows how much functionality will get lost.