Not sure how feasible this is, since it would involve editing a value that never changes (hard to find?) but the Blood Bowl game actually has 9 races, not 8, but it only lets you select from the first 8. Since it does load all 9 races, there has to be a limit in the code telling it to only run through the first 8 when allowing a player to select the race they wish to play.
A trainer option that removed that hardcoded limit would almost be an *expansion* to the game, rather than a trainer.
After browsing the official forums, I read posts that describe the Dark Elf race being incomplete, for an example using Wood Elves models. Plus there might be the option of getting the DE in a future patch/expansion.
Oh I'm certain they will release Dark Elves as part of an expansion - but except for the changed models/skins, the race is fully implemented (skills, player types, star player, logos.. cheerleader models and skins are in, too).
The game has a hardcoded "only the first 8" limit for selecting a race when starting games - and that serves no purpose but to force players to pay cyanide more money (and lets face it... this game was grossly overpriced given the fact that it was a digital download).
If the 8 cap were removed then people will still buy expansions (there remain skills and such to be implemented) but the modding crowd could implement new teams without cyanide's help, assuming players were running the trainer that allowed them to play > #8 teams.
You can't just 'remove a limit' and let the player choose any. The race might exist (if it is complete), but all manner of coding isn't in the game. For example, the game hasn't been coded to allow the player to choose this new race from the menu. So, changing a value from 8 to 9 won't suddenly make a new team available in the menu and selectable.
You see where i'm coming from with this. If the team was available in the game as say, an unlock, then things would be very different.