I’m not sure if WaniKani has changed their stance regarding reviews beyond the burned stage – it always seemed to me that they want to push you away from the SRS towards reading practice at this point. But I would still like a native “Burned Items” extra study mode.
In the meantime, there is the option of using the “Self Study” mode provided by the Reorder Omega userscript with the “100 Random Burned Items” preset.