They also discuss an "eddy current retarder" design, which uses a magnetic mechanism that sounds vaguely like an electric generator but generates heat directly,

They also discuss an "eddy current retarder" design, which uses a magnetic mechanism that sounds vaguely like an electric generator but generates heat directly, as an alternative to the water break/impeller design. Sounds like maybe you get the best of both worlds, in terms of a smaller package but avoiding the motion -> electricity -> heat conversion losses due to the extra step.