The Buddy Hall cue guide trainer (which is an elongated clear plastic tube on a tripod that you stroke the cue tip through), addresses the cue tip accuracy. It doesn't address the elbow drop "directly," but rather indirectly. Like you say, if you have a "bad" elbow drop (i.e. the timing of the elbow drop is bad, causing the tip to rise), this training aid will show that to you. But it won't "address" it.
Tom Simpson's Stroke Groover does, however:
http://seyberts.com/products/Stroke_Groover_Stroke_Trainer-101051-84.html
This is the more portable version of the "strap hanging from the ceiling" that another poster mentioned.
-Sean
I like your photo illustration, and if the Cue Tip Come UP AT STROKES END, and leaves a FLAT Horizontal Plane chances are you are Dropping your Elbow.
I find if I force myself to STAY DOWN DURING THE ENTIRE SHOT NOT MOVING UNLESS I WILL FOUL A BALL. I do not drop my elbow, and benefit by seein my entire shot...Good Luck.