HTP is definitely more than just an exposure correction ("exposing to the right"). Though I don't think Canon have provided a full technical explanation of how it works, it may be a hardware related feature since it affects both JPEG and RAW files and it changes the lowest ISO from 100 to 200 on most (all?) cameras.
"Highlight Tone Priority shifts some dynamic range from the mid-tones to the highlights to produce smoother tones, with more detail, in the bright areas."
The effect is equivalent to exposing the brighter areas at ISO 100 while exposing the darker areas at ISO 200. They could be doing this 100% in software, but if so then it wouldn't need to affect the RAW files and it could be added as a post-exposure option in DPP - and it isn't.