I like the handkerchef, garter, and violet ideas. I got married at age 29, after being out of the active chapter for seven years. Two of my BMs were ADPis and two weren't. We didn't do
anything at our wedding to signify ADPi. DH is former military (different band of brothers), and three of his GM were army buddies. Our wedding was a reflection of our lives together and he would have been very uncomfortable if he were suddenly serenaded by some sisters at his wedding.
Ask the bride how she wants to honor ADPi. Don't spring any surprises on her unless you are
positive that she wants them. If the bride wants violets in the flowers, then she can have them. But don't show up with flowers on the day of and expect the bride to love them (flower arrangements are expensive and I would have been incredibly angry if someone tried to mess with mine on my wedding day).
My point is, everyone has their own vision of their wedding day. I think you're a great sister for getting ideas for the bride, but make sure you suggest them to her first. What you think is fantastic and wonderful and can't-live-without-it for a wedding may be very different from what the bride imagines.
(p.s. IMHO, the last thing you want to do is have the bride borrow badge on her wedding day. She's got a million things going through her mind and keeping track of a badge isn't one of them. A wedding (at least mine) was like a wonderful, slightly stressful, surreal dream. I had no idea where I put anything - lost many glasses of champagne and tubes of borrowed lipstick - and I was getting pulled from place to place, person to person. There is no way I could have kept track of a badge. It was hard enough with a wedding ring and husband
)