My boyfriend's fraternity does lavalier. I don't know whether this procedure is chapter policy or national policy though . . . The guy has to be an active member for a year before he can lavalier (so you can't get in and the next day lavalier), he can only lavalier once . . . ever, he asks his brothers before he does it, general rule of thumb is that since this is seen basically as an engagement by the chapter the couple should have been together about two years (depending on the relationship). They usually also give the lavalier on a special occasion for the fraternity where all the brothers are gathered together, like the founder's day dinner.
As an amusing side note although I haven't been lavaliered . . . yet
I can still wear the letters . . . my brother is a member of the same fraternity and with the chapter's permission I may wear them as a blood relative. I wouldn't feel right about it though, I'd rather wait and get them from my boyfriend.
Emily