Off topic, somewhat.
The bear lockers at trailheads are for people to move things out of their cars parked at that trailhead into the lockers. The bear lockers in the backcountry are for people who are backpacking to store their food in. None of them are for caching food, or leaving trash in, and rangers discourage caching because all too often, plans change and people never pick them up and the cache becomes trash. So yea, you can do it, but realize that it is not encouraged and you run the risks as described of seeing it gone for various reasons. It is in fact ILLEGAL to leave a cache in a bear locker. There will be signs on the inside of the locker door telling you just that.
Using the right food can mean the difference between needing a cache and not needing one. It is entirely possible to get ALL your food in a bear can -- if you repackage everything into ziplocs. Using the bags the Mountain House meals came in makes them bulky. Using the cups that cup a soups come in, throwing in canned goods, attempting to bring breads of whatever kind and keep them from getting smushed, bringing whole eggs or fruit -- all wastes of space in a bear can. Use couscous instead of pasta shapes like bowties or spirals. Don't bring apples, bring dried apples. I've seen folks bring food from the kitchen and waste a ton of space, turning the HUGE bear cans into kegs of heavy, bulky food. If your trip is less than seven days you can manage even with the Garcia.
http://sierrawild.gov/bears/how-pack-bear-canSo no, it's not off topic exactly.
Google the Wild Ideas Bearikade and you can avoid caching altogether.