How do I make consistent landings from the (instrument-less) backseat?
Since nobody else has posted an answer, I follow Dan's suggestion and copy my comment here. Unanswered questions make this site look bad.

Practice, practice, practice. It's for a reason that the pilots who fly consistently good landings are the older ones. And remember where the horizon is in reference to the airframe and how the engine sounds. At approach speed (= at high angle of attack) the aircraft will show large attitude changes for small airspeed changes. Remember how high you are at conspicuous points along the approach, don't be shy to look sideways for this. And keeping a little throttle makes better landings, so don't feel negative about this.