im not sure if they can do it, but there is a russian guitar company by the name of shamray that do full custom guitars, one of there workers posts on guitar.com, you could ask if they would be willing to machine you an 8-string trem, that being said, your definatly not going to be able to fit 8 strings in the space of a 7-string neck since the 8th string is a bass string (also you had better be using 2 truss rods, just a heads up for the future) good luck on it though dude!
also unless your going to be using a 'blade' type of pickup up i dont think you will be able to use 7 string pups, shamray also does custom pickups, but like i said there in russia, i would suggest going to guitar.com and making a post addressed to rockandroller since he would know more about the custom shop then i do, i plan on ordering a custom neck from them soon but there a bit backed up and its about a 6 month wait right now.