who told you you can't put live plants in a tank that is cycling??
Plants will soak up the ammonia if they are fast growers like hornwort.The ammonia will not hurt the plants. I have just fishless cycled with ammonia and my plants are fine.
People who are really into planted tanks say you can add the fish sooner with lots of plant matter in the tank.
Check out the
www.plantedtank.net website for more info.