As expected, the problem was due to my noobishness with regard to OpenGL/GLSL. I'd assumed (in error) that texture coordinates being uploaded in pixels inferred that GLSL wouldn't follow the typical 0...1 standards for texture sampling. Kinda silly in retrospect. Output is a little stuttered, but I'm also using Intel HD Graphics on a three-year-old laptop, so whatevs.
zsbzsb and Laurent - I won't say my introduction to GLSL was
painless by any means, but with your help it was at least palatable. Many thanks!
Sample output using exponential zoom, warping, color dampening, and a square for input is attached. Unfortunately I can't upload a video displaying how awesome this looks (re: Intel HD Graphics).