Rhubarb like many other things we eat contain Oxalic Acid which is poisonous when too much is eaten. Even the stalks have it which when having eaten too much of it gives the "Spring Cleaning" effect.
"From an MSDS (Material Safety Data Sheet) for Oxalic acid, LD50 (LD50 is the Median Lethal Dose, which is the dose of a drug or chemical predicted to produce a lethal effect in 50 percent of the subjects to whom the dose is given) in rats is 375 mg/kg. So for a person about 145 pounds (65.7 kg) that's about 25 grams of pure oxalic acid required to cause death. Rhubarb leaves are probably around 0.5% oxalic acid, so that you would need to eat quite a large serving of leaves, like 5 kg (11 lbs), to get that 24 grams of oxalic acid.
Plant leaves, especially rhubarb, cabbage, spinach, and beet tops, contain oxalic acid. Oxalic acid is also found in potatoes and peas. http://www.rhubarbinfo.com/poison "
I think the leaf tea would work quite well and don't think that it should get a bad rap for being so dangerous not to grow.
A little off topic but the first thing I though of when I saw this was - cabbage moths.
http://www.bugasalt.com/
I had to get one. I'm looking for alternate ammunition like sand or maybe sugar for the garden. Looks like it's going to be a ton of fun.
Was looking for some of this natural pest control products, and in Mexico some people use chilli spray
http://www.annettemcfarlane.com/Doityourself.htm
Hope this links proves usefull for you
Haven't had much trouble with pests luckily, but I've read that if you put a saucer with some beer in it, the slugs can't resist it. That, or half and orange put cut-side down supposedly attracts some bugs. Lift it up after a day or so and throw it away.