If it is truly made out of tar, then I would think that some really nasty, toxic, and flammable organic solvent like toluene (with a little elbow grease and a lot of ventilation), or something a little less toxic like mineral oil might get it off (with a whole lot of elbow grease). However, those may very well attack any hoses under the hood, paint, and anything else they may come in contact with. I did have a patient a few years ago who was putting a new hot tar roof on when he fell into a puddle of hot tar, and we used all the mineral oil in the pharmacy to remove it (painfully slowly, literally and figuratively, but it did work).
I'm not a mechanic, but I do have a degree in biochemistry. The old saying from organic chem was "like dissolves like." Tar is a high-molecular weight organic compound, and the less polar the solvent (the less like water) the better it will work to remove the tar. Unfortunately, some of the best organic solvents are extremely toxic, carcinogenic (may lead to cancer), and teratogenic (may lead to kids with six fingers per hand, two heads, three eyes, etc.), not to mention bad for the environment when released.
Oh, I forgot that some of the really good organic solvents also love to attack plastic.