That's what I did and it works great. I even filled the dud tub with aspen so the probe is sitting where the snake would be..
Only problem (which isn't due to probe placement), is that depending on how tall the rack is, your temps could be slightly different from the top to the bottom. Hot air rises (or vice versa idk), either way the very top could be a couple degrees off, which is really no biggie. Just keep an eye that it doesnt get too hot or too cold.