I can only reply from what the midwife who did my antinatal classes told us.
She tried it about 20 years ago, got severe diarrhea and vomiting (at the same time). She went into labour the next day and the ob told her there was day old meconium in the amniotic fluid, which was caused by the castor oil.
This is 2nd hand info, but she told us not to try it.