Legal dressage bits can be viewed from the United States Equestrian Federation (USEF) Rulebook. Select the downloadable PDF for the "Dressage Division" to view.
Many of the bits listed for dressage do not offer much tongue release. The straighter the mouthpiece, the less tongue release. So the Mullen-mouth snaffle or unjointed snaffle will have less tongue release than the traditional single-jointed snaffle. In some cases, horses may even reject the larger, soft rubber mouth snaffles, so experimenting will be necessary.
The type of noseband (dropped, cavesson, flash, or crossed-figure 8) you use may also be a factor in how the horse accepts the bit.
Determining the best bit will require a little "trial and error." If you have access, you will likely find advice from a trainer or clinician to be valuable as he/she would be better able to determine suitable alternatives to try when observing the horse in action.
