I agree with VASH in that for a public server it would be unpopular. I would envision this mainly for organized competitions such as clan battles where people play from start to finish. For a general server where people come and go mid-game, incoming grunts forced to face off with specials may turn around and leave or spectate till the next map rather than try to fight their way up the classes.
While I like the concept and think it has merit, unfortunately there is no provision in the PS2 version to lock the sniper, rocketeer and engineer classes. To the best of my knowledge it can't be done.