you're absolutely right, that one was way ahead of its time. good thing this one actually got a concrete ending.
So hibari-kun and no bra actually aren’t considered part of the Otokonoko/trap genre because the genre wasn’t around when they were written. So I went digging for the earliest trap manga on mangadex that fits the criteria listed by by tvtropes. The trap boom started with Bridget, who is no longer part of the Otokonoko canon, so that eliminates series before 2002. Additionally, the target demographic must be Shōnen or Seinen. This eliminates w Juliet from the running. Tvtropes also eliminates Maria Holic from the running for reasons I’m not sure of. From there it is a matter of how high the standard is for being “about” traps.
I think pretty face(2002) fits the bill pretty well, but one could argue it is more genderswap than crossdressing. in that case, the next one up would probably be Mai Otome(2005). Mai Otome has enough plot elements, going by the blurb, that maybe someone could argue it is not properly about traps. The same is likely true for idol A(2005). If anyone is truly that much of a purist, I would say Maid in Japan is uncontestable.