If looking to simply stretch the lats, just grip a chinning bar and hang for a few seconds with your arms as relaxed as you can make them. You can also perform a triceps stretch against the wall by raising your arm, bending at the elbow as if you were going to perform a unilateral db extension. Place your elbow against the wall and lean in. If done properly, this stretch can be felt in the tri's, delts, and lats.
If you're after a stretch during an exercise, I do this all the time with pull-downs and pull-ups.
Firstly (if you don't already), try using straps. I use straps on every working set of rows because they allow me to relax my grip a bit, which transfers more tension to the targeted back muscles. "Relaxing" the hand and arm muscles also permits something of a stretch on your lats when the bar is at the highest position - similar to if you were to just hang from the bar, but this stretch is achieved between each rep.
I hope I explained this clearly.
It's what I like doing, and I at least partly attribute it to my back, which has always been about my best-developed body part.
