I tried castor oil with all 3 of my first babes, and all I did was poop. I know that's not what you want to hear, I'm sorry.
The verbena oil cocktail I had the other day has castor oil too, but because of the other ingredients I didn't spend any time on the toilet. The recipe is pretty googleable, or your midwife might have her own ideas.
Alternately I just heard of castor oil belly massage. Apparently it worked for a friend of a friend.
Good luck! I'm so sorry you're feeling desperate. The end is soooooo hard.
I have heard both stories of it working wonders and horror stories of women with cramps, diarrhea, vomiting and it not even working in the end. ::hugs:: If anything I would look into a cocktail like the other women have said. Also, have you talked with your MW about possibly black/blue cohosh to naturally induce labor?
I started castor oil yesterday, labor started at 1pm...strong and hard. Midwife came and I was dilated to 4 cm. Labored all night, got in my birth pool with very strong contractions. At 3 am they stopped so I got out of the pool and laid in the bed. Woke up at 5 am to find that my labor stopped. Completely. I went downstairs and woke up my midwife and cried....angry and frustrated. Here I am 11pm....whole day of no labor. Every hour I have a contraction that brings me to my knees...but nothing more.
I will never ever suggest castor oil, or try that again
Lots of Like tolovemercy said, soon you will be holding your sweet baby and definitely take care of yourself in the meantime and get nourished and keep hydrated. You are in my thoughts! (and the rest of you mamas who are also waiting to meet your LOs!)
I am so sorry. Your baby will come out. Soon. I promise. 
In the meantime, please sleep, rest, and eat well. Your body needs it. Thinking of you!
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