Schraf,
I spent the last several hours searching through medical research journals looking for answers to some of your questions. The first thing I learned is that I am really glad to be an invert researcher - vertebrates are just gross on the inside!
Unfortunately I could find very little information on the effectiveness of castor oil packs (not counting naturopath resources, looking through scientific literature). A general web search revealed thousands of websites, all of them naturopath, herbal, and homeopath sites. It was not a confidence boost IMO that pretty much all of them cited Edgar Cayce as the original source of the procedure. Him having been either insane or, more likely, a major fraud does not make me trust his medical expertise!
I did find several toxicity studies, but these are based on oral applications. Interestingly enough I did find that Mussolini used massive doses of castor oil to torture prisoners (sometimes mixed with gasoline to insure fatality). Dermal applications are mentioned in the literature but only as a means of administering lipophilic medications. I did find skin application toxicity experiment with rabbits that resulted in relatively mild skin irritation (they were looking at castor oil as a base for cosmetics, apparently many people have allergies or other reactions to these cosmetics (Rantuccio, et al. 1981).
Reading through some of the more prominent natural healing cites raised several red flags for me. The first, and most important (short of using Cayce as an authority!) is that the castor oil packs are reported to cure just about everything. I am suspicious of supposed cure-alls, the pharmacology alone would seem to require magic. The second major red flag (and one you hinted at) is that to work on whatever organ they are supposed to the packs need to be held over that organ (kidneys, liver, stomach, etc). This makes no sense, either it penetrates the skin and gets into the bloodstream to get to the organ in question (so it doesn't matter where it is applied) or it somehow penetrates straight down through skin, multiple muscle layers, bones, etc to reach just the organ underneath.
So, I am sorry I have nothing profound but I hope there is something of value.
An oral toxicity study:
http://ntp.niehs.nih.gov/ntp/htdocs/ST_rpts/tox012.pdf
A skin toxicity study:
Rantuccio F, Sinidsi D, Scardigno A, and Coviello C (1981)
Histological changes in rabbits after application of medicaments
and cosmetic bases. II. Contact Dermatitis 7: 94-97.
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