What about branded jewellery related items? They aren't worn in a way that could alert the baddies, but may be seen in safer situations (with friends and family, in a jewellery shop).
A polishing cloth with branding on it would fit into postage box easily and would be relatively inexpensive to produce to either give away or sell. A little reminder of DBL when you take it out of your handbag for an on-the-run polish.
A triplet loupe with DBL on it would be more expensive to produce, but desirable to purchase. And you could put the Diamonds by Lauren words (and diamond shapes?) on a flat cord to wear around your neck when using the loupe. Or even a short branded clutch cord to help you find it in your bag.
DBL has lots of fluorescent diamonds - what about branded UV mini torches or torch keychains. A good way to to show off those live fluorescent diamonds and talk about DBL at the same time.
A branded keychain is a bit prosaic, but would be relatively cost effective to produce and wouldn't be as much of public display of diamond ownership as a T-shirt. Or make a diamond encrusted platinum keychain....... and use a little coloured pear diamond in the appropriate spot. Although that might be a tad excessive
You could also have a logo on small jewellery travel case or pouch. I think the DBL logo should definitely be on the silk on the inside of presentation boxes.
I think DBL is an elegant company, and my feeling is the DBL style would be best preserved with subtle and appropriate branding.