if it's matrix input, you can use the mouse coordinates from clicking on the jit.window (output the dump out) to calculate a getcell message to the input matrix. you'd have to convert mouse coords to matrix-cell coords, but that's simply two scale objects.

if it's gl input, you'll have to readback to a matrix using jit.gl.asyncread (for faster asynchronous readback) or jit.matrix (for slower synchronous readback).