I have seen an enclosed esc done twice before with success, silicone is not conductive to electricity and in liquid form has considerable cooling properties.
http://monster.traxxas.com/showthread.php?t=469208
Some university students actually submerged an entire computer and power supply into an aquarium filled with pure silicone just to prove that it could be done. I have 2 pairs of 3s lipos to run my slug - if my soldering skills hold up. If I cannot remain waterproof then this whole project looses a major benefit.