[DCP-o-matic] Edit bitmap Subs (VOB, etc.)

Not strictly related to DCP-o-matic - but does anyone know a way to edit bitmap subtitle streams in e.g. VOBs from a DVD? DCP-o-matic allows to change size, position and color. Wondering wether it was possible to add a list view - and then, as once proposed for timed text, allow to edit in/out times, shift titles, erase individual titles? Wondering wether there could be a way to turn this all compatible with PNG subtitles? After all, there is not so much difference in handling between bitmap subs and PNGs. Would be cool if both could be dealt with through the same interface! - Carsten