Most coasters, especially multitrain types, have brakes available throughout the route. Sometimes these are to make sure the train is not exceeding safe limits for the track ahead. Energy returns on those systems are probably pretty low.
Some of the older coasters have multiple circles of track around the ride with bumps and dips to use up the energy before entering the station, but a lot of the newer coasters, taller and faster, wedged into tight footprints (Top Thrill at Cedar Point comes to mind) definitely could recycle some of the braking energy to reduce the load needed to hit 120mph in 4 seconds. The braking at the end is pretty severe.
However it would allow some enterprising person to make a HDCP intercept device which would allow a person to DVR supposedly protected content, by grabbing the HDCP stream. So comcast (among others) could not stop you from recording live HD content (or pre-media release PPV content) by setting the "do not record" bit.
Or it might make a nifty media converter so I could use my older, non HDCP HDTV (they do exist) with new content.