It's sound in theory, but in practice the difficulty is in getting the target lines exactly the right distance apart. If you're off by even a fraction of a mm, the target no longer provides a correct infinity reference.
What's wrong with just aiming the camera at a target with well-defined vertical and horizontal lines, located at least a half-mile or so away... a TV tower, for example?
(Tip: Once you know your RF is correctly lined up at infinity via the above method, you can set it up on a table in front of a white card, shine a laser pointer in through the eyepiece, and mark the locations of the laser dots on the card. This will give you a "portable infinity" that you can use in the future for quick checks.)