This has been my experience as well. To LH, the concept of standing by for a flight other than the one you are ticketed on is completely foreign. And remember, this is Germany where rules, rules, rules, well... rule.

Having said that, once (in probably 8 attempts or so) was I successful in getting on an earlier flight instead of schlepping for two extra hours at FRA. It was Christmas season, the day before my birthday, sunny outside, everyone was in a cheery mood, the agent I spoke with must have been related to Mother Teresa, and I was wearing my lucky blue shirt. So YMMV. That day at the FRA RCC, I kindly asked the agent if she could connect me on to DUS on the earlier flight (it was a UA ticket, LH metal). She said she didn't think so, but somehow mustered the courage to try anyway. Called the gate of the departing flight and (surprise!) actually knew the agent personally. They chatted for a while, laughed and ba-da-bing I got my seat on the flight.

Who needs luck???