It might be worth trying a different agent as I think "cross-pollination" on Star carriers is increasing. Not sure how strict LH is but Air New Zeland also reserves exit rows for its Premier members, but recently let me snag one in advance with United 2P. (Although I was actually at the city ticket office and was looming over the agent at my usual 6'6"
). Speaking of which, if you are tall, I've found that a politely worded fax to the airline explaining that you are among that freakish percentage of the population that is over six feet has always secured me an exit row. Even on United when I didn't have status.