Dunno 'bout Eragon, but in Jurassic Park they substituted in a digital double, at the point where a man is snapped up by the T Rex. This was a wide shot at night so you couldn’t pick it.
Typically any King Kong movie is shot with large green-screened fingers holding the actress in peril. These are keyed over with the model Kong and the moves are matched to the live plate.
You could of course reverse the match as you say (place your keyed person as a texture on a plane, parented to the dino) but the ballistics of a person on a Dino (or other large beast) wouldn’t match correctly. i doubt that you could really talk an actor into being hurled around 10-15 feet through the air! But good luck if you can.
If it were me, I would construct the scene to occur in the dark with most shots in wide (worked for Spielberg!). Use insert or close up shots from a low angle looking up at the actor, this will hide the lack dino beneath. you could even render a riders point of view shot from on top of the dino, thereby eliminating the need of a rider in shot alltogether.
VFX are about tricking the audience, good luck.