Well, the trouble with that idea is that it is far too elaborate a ploy. The chances that Hobbes would not only be accepted by Confed, but indeed allowed to serve in the military seem extremely slim. I'm sure the prospect of losing Hobbes wouldn't have bothered Thrakhath, but would he have seen the point of doing anything so complicated without gaining anything from it? Besides, there already was a plan to hurt Blair, through Jazz and the Mandarins. Hobbes ultimately wasn't used until much later - and indeed, on the losing path of WC3, it's possible for him to die without the trigger ever being activated.