mpv does this (must enable manually for SSA subs). In VLC you can also add the black bars to the video with a filter so the subtitles draw on top of them, but some renderers won't like the aspect ratio change.
An annoyance is that deliberately absolutely-positioned subtitles (e.g., translating onscreen text) are shifted too -- more ideally the feature would only apply to subs without inline margins. Similarly, the time delay from in the sibling thread is undesirable for subs that are timed exactly to what's onscreen, though that's much less programmatically identifiable.
