Not sure  about  who offers what,  but a simple  blockage of the  exhaust flow is  what I'm thinking.  Jake offers both  their true  engine brake system that   works  with the  valves,  and turns the whole engine into a  compressor,  plus  they are offering a  simple  exhaust brake that  simply   restricts the flow out the manifold.       

This may be more complicated  than I thought..      The brake  would need to close   the instant  revs  start above a certain point,  and  open again  soon as  I  hit the bottom of the canyon..  I'd probably need a third arm just to run a mechanical  butterfly.. Or  Something would have to tie- in the governor to the  brake controller, so they aren't fighting each other.

Maybe a simple switch on the throttle lever,  which activates the brake  @ idle?   Isn't that how  the add-on kits work on diesel pickups?